In a world I'm designing, a group of unknown animals evolved into the fearsome Cipactlis. Some traits of these Cipactlis include: having limbs in the stance of toads and wrinkly skin, the tail fin of a fish, but with more crocodilian traits, like armored skin, a crocodile-like head, and a crocodile body, have retractable claws, are predators, with the males and females being 26 feet long, live for 80 years, store food in the winter, have no extra mouths, live in large groups called floats, which eat large and very numerous prey like large fish and gazelle and elephants and other mammals or birds, and they are cold blooded. Given those characteristics, what species could they have evolved from and what evolutionary pressures would lead to them?
These Cipactlis could be crocodilians that are better adapted to aquatic life. The fish-like tail fin would be useful for an aquatic creature. Paddle-like hindfeet like a toad may also help them to swim through the water. The toadly forelegs may be for grabbing onto giant fish, which its long, sharp claws could then kill. The claws could also be covered in a sheath, to protect them from attacks which don't require them
Well, every characteristic needs to have a reason for existing in your creature and have given an advantage, even if that reason is no longer there (like our appendix). This even include behavioral traits.
So the first thing you should look at is which characteristics of fish and Toads and Crocodiles you want. Make sure you don't get traits working at cross purposes. Some frogs and toads can absorb some oxygen from water through their skin, but that won't be possible if you want a tough crocodile hide.
Retractable claws in a predator would make sense under certain circumstances. If I recall correctly, a Cat's claws retract for purposes of stealth and so as not to get blunted with the action of walking and running. Maybe they need to be long claws to help take whatever prey they are after, but the shores the come up on are really hard on the claws.
Some things you want to think about is what is the prey, is it large enough and numerous enough to support a really large predator that lives in family groups.
You may want to consider the floats and food storage to be a winter or hibernation technique, with the float breaking up and dispersing into smaller groups during the warmer seasons. A large group of large predators will eat a lot, and if they are all concentrated they will clear out the food sources pretty quick. It's why so many predators tend to be solitary or small family kinds of groups. This can be mitigated by being cold blooded and therefore not needing as much energy.
The hairy frog, Trichobatrachus robustus, creates retractable claws by breaking the bones in its feet.
Many frogs begin their life in a tadpole stage, eventually adding limbs and losing their tail as they age. If there was an evolutionary advantage (such as spending a large portion of their life swimming), the adult form could keep the tail, although it would be different from a fish tail.
Toads' lumpy skin is one of the main features that sets them apart from frogs. The skin is much tougher, giving them a layer of defense against predators and making them a lot more resilient against drying out. This means that toads can live in much drier climates than frogs, such as Australia, where cane toads are a prolific invasive species.
Cane toads are very dangerous to an ecosystem for two main reasons: First, they are omnivores, willing to eat many different things, including plants, insects, and small animals. Reportedly, some toads will die by trying to eat things that are bigger than themselves. Secondly, cane toads secrete a smorgasbord of powerful neurotoxins through their skin. Many animals that eat the cane toad will die.
Amphibians are all ectothermic, or cold-blooded, and some can live for decades. The olm, for example, has an average lifespan of 70 years. However, long lifespan is usually associated with creatures that are large and have a slow metabolism. Creatures with slow metabolisms generally move less quickly. Ectotherms, such as reptiles and amphibians, are sluggish in cold environments, which make them less dangerous.
One way around this slowness is to use ambush predation. This is the very effective hunting strategy used by crocodiles: You wait and stay hidden until prey gets close, then bite it. However, ambush predators gain no advantage from grouping together and rarely eat prey bigger than themselves.
In order to make your animal realistic, you should consider what environment they live in and how they catch their prey, and then choose traits that are optimal for that lifestyle.