Could genetically engineered horses make cavalry survivable on a battlefield with modern technology? While machineguns made cavalry obsolete, could there be a way that horses could be modified to be useful again? You would also get of one of the main issues of cavalry solved as you wouldn't have to worry about horse breeding making war horses rare. This cavalry can use both guns & any melee weapons. (Effectively the only thing that used to keep infantry from being destroyed by cavalry charges was formations but modern infantry doesn't take up formation so a cavalry charge would be effective against modern infantry if they didn't get annihilated on the way there.) This genetic engineering is limited by real world constraints. The tradeoffs between size & food consumption are still there. If an organ is included, it has to exist in a already existing animal. If another animal would be a better baseline for this, it can be used.
Armoured cavalry is faster than horses, carries self-propelled guns, is more capable of traversing most terrain, and offers a much higher combat value-per-unit than any animal could.
You could make horses that ate half as much and spat acid, and they wouldn't come close to the utility of a tank brigade in modern warfare.
Change the battlefield:
Frame challenge: To make horses practical on the battlefield, you need to change the nature of the modern battlefield. I think Nepene Nep covered good engineering for these horses. Short of giving intelligent hexapod horses functional arms to carry weapons in, you aren't going to change horses enough to make them practical on the field.
- Man-portable super weapons: If every individual soldier has a relatively cheap, rugged weapon that can destroy any vehicle on the field, then planes, tanks, and possibly eventually even trucks and jeeps will become an expensive and wasteful investment. There will still be a need for transport, but weapons will make the form of transport as cheap as possible. Trenches would be ineffective except as a fixed and vulnerable means of concealment. My favorite weapon in this case would be some form of mini rocket launcher with various types of specialty munitions, but lasers, plasma guns, and the like are all fodder for these weapons.
- EMP: If these weapons (or related weapons) unleash massive amounts of electromagnetic interference, then drones and robotic weapons will be rendered increasingly worthless. even vehicle ignition systems will be rendered obsolete, and trucks would need to move troops to the edge of the battlefield, then drop them there or risk being incapacitated. Even missiles today are extremely dependent on electronics, and would need long "dumb" portions of their attack to overcome abundant EMP attacks. Surveillance and communications on the battlefield would be operated by simpler and simpler systems as even radios were destroyed.
So to actually carry men faster on the battlefield, or move supplies about in and quantity, you would need horses (or something like them). Infantry on horses with hit-and-run tactics would be more mobile than infantry. Any equipment in the direct field of conflict would get hauled by a horse or a horse-drawn cart.
Horses are still used in primitive conditions (especially in asymmetrical warfare) for hauling and mobility, but can't compete for these functions with modern equipment. So get rid of the modern equipment and see what happens. What is old is new again.
Yes, however not as cavalry but as Dragoon type brigades.
Armies of today live on money, fuel, manpower, mobility, fuel, spare parts, money, skillful training, maintenance, production capcacity and money.
Thats a lot of money, but also a lot of fuel. Especially an area like europe if it was ever cut off from fuel import would have little capacity to create its own.
Using genetically engineered horses to reduce fuel consumption while still creating mobility is a good idea. Rather than replacing existing vehicles, they would simply be additions to currently un-mechanized infantry. The infantry would use the horses to get around faster than they would without vehicles and retain their stamina for a fight, while also carrying more supplies. The horses can be genetically engineered to eat just about anything, from grass to leaves and bark to human waste. That way they can survive for longer without needing specific food supplies.
In combat the horses are used as Dragoons: you ride them out of visual range of the enemy, then dismount just before you go to fight and leave the horses behind. This means each soldier can bring more supplies for combat, they can reach their destination on the battlefield faster and they have more stamina for the actual fighting. The horses are genetically engineered to be smart enough to recognize uniforms and not need tending while left behind. That way horses could hide themselves from danger, look for food or react to the soldiers calling them back should they need them again.
Edit: since any organ from other animals is allowed, to make the horses more resiliant to shrapnel and bullets you could use spider silks. Spidersilk could be made within the skin of the horse, where the surrounding cells will pull it between them allowing the strands to migrate and reinforce the skin. Spidersilk farming takes a lot of time and effort which makes it rare, but with the addition of many spidersilk glands throughout the horse's body and the fact that it will likely be a few years old before it is used it has time to create a reasonably thick layer of added spidersilk protections.
Add in other skin types, badger and bear skin both is thick, loose and tough. Bears can hit each other hard with sharp claws without causing significant damage, and Grizzly bears are notorious for their resistance to small-arms. This would also help the horses survive getting hit, although the most important word is "survive", not "remain fully capable of continueing". This is in no way a free pass to survive sustained fire, even if the skin was 100% bullet proof the sheer concussive impact of a machine gun would quickly incapacitate and eventually kill the horse. You could improve their ability by making them Grizzly sized animals but that would also mean a larger, more food consuming animal that would unlikely have the niche you want.
Yes, because machine guns aren't the top counter to horses.
Infantry are worse at handling machine gun fire than horses because they're slower. Only very heavily armored trucks can handle machine gun fire.
The main reason why armored cavalry replaced horses is because they have greater strategic flexibility. Horses require lots of water and food and die of exhaustion and while you can go 30 miles on a horse in a day, you can go that far in a few hours on a decent road in a truck, and they just need a load of gas to keep going, or a few replacement parts.
Horses also require riders to spend several hours a day grooming and feeding and training horses every day.
When trucks became much more cheap, it just wasn't financially worth it keeping horses around.
As such, there's a single simple modification that could make horses more valuable today.
Make them much, much smarter, loyal, and fearless.
Horses who were intelligent and loyal could clean and feed themselves. They could forage for food and train to be better hoses. They would be like human soldiers, but better. This would reduce their cost a lot, and give them greater flexibility.
Them being fearless would make them much more willing to ride through gunfire and artillery shells.
Make them able to eat a wider range of food.
Some alterations to stomach fauna would broaden their ability to survive on whatever foods. This would allow them to feed in hostile environments where it might be hard to get petrol for vehicles.
In terms of their role, we have long since moved past the time of infantry charges. Now small squad warfare is the norm. You have small flexible squads of soldiers that can work together to use cover, ambushes, and covering fire to defeat enemies with less casualties.
Horses with their immense agility would help do that. They could ensure machine gun ammo was in the right place, or quickly move a sniper to a safer position. They could fire at people from a position humans could never reach in time.
The goal would be to use their superior speed and tactical flexibility to hurt enemies more. They'd be of most use in places with bad roads or urban environments where pure speed and distances were less of a concern and places where air superiority is not assured so you can't just scout with a plane.
No, not cavalry
Modern combat tactics are mostly about stealth because offensive power has (generally) outstripped defensive power. This is visible in modern infantry doctrine, which revolves around small squads moving from cover to cover to gain a tactical advantage before engaging or providing intelligence that let them direct more powerful weapons like CAS or a mortar attack. Basically, there are very, very, few situations in modern military combat where you can see the enemy and you can't almost instantly kill them or be killed by them.
This means that cavalry wasn't phased out because horses are vulnerable to machine gun fire, it means cavalry was phased out because the strategy is incompatible with modern military doctrine. A cavalry force is a large mass of soldiers who ride against an enemy in battle, but who are they going to engage? It's not like there will be a couple hundred or thousand enemy soldiers conveniently lined up in a field somewhere for the cavalry to run down. No, they'll all be in small squads, spread out and hidden, and ready to blast the highly-visible cavalry group away.
That said, the modern military still uses horses, particularly in special operations environments where mobility over extremely tough terrain is required and large loads need to be transported stealthily. A genetically engineered horse could fill that role, and there's a reason that the military invested millions of dollars into robotic pack-animals that combine the best of both worlds: easy handling and care along with all-terrain legs without the detrimental psyche and animal needs of a horse and the inflexibility of wheeled vehicles:
Limited by real world constraints? No.
Horses are big targets that aren't good at moving quickly other than in straight lines, and getting shot one time is enough to seriously reduce their effectiveness. Now, consider that these days, every infantryman carries around an automatic weapon, and consider the fact that there is no genetic modification that lets a horse survive getting shot multiple times.
They work better for special forces rather than cavalry.
Now, if we had supertech on hand, this might be possible, but the resultant creature wouldn't really look or act like a horse anymore.
To expand a bit on @DWKraus's answer (better individual weapons): Turns out metal is incredibly easy to detect, and large masses of metal (say, anything larger than cell phone) aren't really viable on the battlefied any more. Man-portable smart missiles made primarily of plastics have removed essentially all vehicles from the battlefield (and probably all squad weapons to. Maybe you don't even want to carry a pistol any more...); the missiles have enough smarts that you just aim them in the vague direction of anything and it's going to get hit and destroyed. Valiant efforts by tank units to do point defense with their own missiles were unsuccessful because the plastic missiles are great at killing metal objects, but not great at killing each other.
You're going to want transport still. Wooden wagons, horses, etc. The horses probably still won't get used as cavalry (as in fighting from horseback), but fighting as dragoons (dismount to fight) is extremely common.