How could this cave-dwelling draught animal evolve?

Question

This animal is a domesticated animal that was originally found in caves, and is still used there as a draught animal. It is a horse-sized troglomorphic tetrapod that is capable of pulling its own weight on a wagon. They carnivorous, and are usually given a diet of undesirable pieces of meat, which is supplemented by rats and other such pests, and any other animal part that their owners get but don't want to eat. Their pre-domesticated ancestor was quite similar, and had a diet of mostly cavefish, though it will eat any meat it could find. What could this animal have evolved from, and what pressures would cause it to evolve in this way

Answer 1

When I was brainstorming ideas, I came up with various creatures that live underground and what their comparative qualities were and what would be best for it's niche. Clearly, there isn't a lack of food in your cave system for a species to naturally evolve to a large size. But being big beyond a certain size isn't so useful to a cave dweller unless perhaps there are predators. It wouldn't be strange for it to be cold blooded, as the environment it is in has a fairly constant temperature. A cold blooded metabolism also allows it to use less energy and meals last longer, take the alligator for instance.

Being tall isn't that useful in a cave, so I would guess that this species would be much lower to the ground than a horse. Also, being able to squeeze into tight spaces is beneficial, so not being too wide either would be good. I'm thinking something with the body proportions of a lion or alligator (although alligators are kind of wide) would work.

Troglodyte Mammal Concept

This concept is based on the naked mole rat. It is an interesting creature. It is practically the only cold blooded mammal. I thought the idea of a mole would be good because it could be powerful enough to dig burrows, so might be able to be used as a work animal.

Sensory abilities:

If it spends most or all of it't time in darkness, it might adapt with smell, touch using whiskers, or using clicks and hearing to map out its surroundings. Perhaps they could even have a heat sensing organ like the pits on a pit viper's nose, which can detect nearby body heat. It may have poor or nonexistent eyesight, and pale skin. When in the water or wet soil, various detection methods like vibrations or even bio-electroreception, (sensing the electrical impulses of muscular contractions of animals) might be used. [Note: electroreception doesn't work in air] Also, being able to sense the planet's magnetic field would allow them to easily keep their sense of direction even without sight.

Physical Attributes:

I imagine an upsized mole like animal would have powerful jaws, tough teeth and muscular forelimbs with which to dig with. The back legs wouldn't need to be as muscularly developed. (I believe that naked mole rats use their teeth to dig, but I think that forelimbs might be more powerful in a scaled up larger creature? Not sure, but that's what I'm going with)

Cats have floating shoulders that are held in place by muscle and not firmly rooted to the skeleton. This allows them great shoulder mobility and the ability to squeeze into tight spaces, which would be useful for an underground species. However, floating shoulders are not usually so good for a load bearing draft animal. This may not be that large of a limitation though, since you said that they would be pulling the load rather than carrying it on their back. This could be achieved though with a specially designed harness, which I will detail in the next section.

I imagine the size of the creatures to be about the size of a lion or tiger, as that is a good balance between strength weight and being compact enough for a burrowing creature. After all, the larger the animal, the larger the burrow holes need to be, which uses a lot of energy, so being roughly tube shaped and not TOO large would be to their advantage.

They would want to have a flexible midsection for following turns in tunnels and powerful forelimbs on floating shoulders would be good for digging and squeezing into small spaces. Their teeth would most likely be suited for whatever diet they had. The neck shouldn't be too long, as you want the claws to easily reach in front of the face. As for tails, I don't know a reason they would need them, but it probably wouldn't hurt to have if they did.

A slow metabolism allowing it to make use of infrequent meals would be good, allowing it to save energy and only rev things up when needed. This goes hand in hand with being cold blooded. Also, a lower oxygen consumption, as mole rats have, would be useful. Mole rats are also social. These animals living in small packs might mean that they are more intelligent and easily domesticatable.

On Use:

The use of a draft animal is heavily dependent on the harness. It allows the animal's strength to be properly harnessed. In the case of floating shoulders, you would want a harness to loop over the neck, with an inverted triangle piece across the chest, with the straps leading under and between the legs. This results in the forces bearing downward where the neck meets the shoulders and spreading out some of the forces over the area of the chest, without pulling on the arms. You would want the area above the neck and shoulders to be stiff, wide and padded, almost like a saddle, to spread out the load.

Answer 2

Cave salamander.

https://upload.wikimedia.org/wikipedia/commons/4/48/Cave_Salamander_(Eurycea_lucifuga).jpg

Because that is the only cave-native tetrapod that has a body plan remotely similar to a horse. I propose these things are as big as they are because they use their strength to move boulders and dig, excavating food in their environment. Which must be some serious D&D worthy caves to sustain a creature like this and Ichthys I get the idea that you like to stay closer to what is real.

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Caves are resource and calorie poor. Cave dwellers conserve calories. An olm might not eat for years. The capability of sustained effort needed to pull a cart is rare even among mammals and birds and I do not think it exists in any cold blooded land animals. Certainly a cave dweller might be capable of a burst of activity abut then it will rest up for a few days, or months.