When Callum had nothing to do, he always returned to Leo. The place where he had hatched, and also the place where he felt most at home. Whether he felt like practicing his magic among the heat of the volcano or swimming with the fish in the bay, this cave just had it all. But what he enjoyed most was the variety of prey that could be caught here. You see, Callum was a rather picky eater. Not picky in the sense that he only ate very specific foods, rather, he insisted on eating something different for every meal. He gets tired of things very easily, and luckily Leo had just the right supply of everything so that if he rotated everything in his diet just right he would never even have to leave his cave. On the food side of things at least. In terms of adventure, he left often to see new sights and find things to investigate.
But he had had enough of his adventuring in other caves for a while, especially after that meadow deer incident. Although he had to admit the gathering in Orion had been rather nice, and he had gained a stone too! Although he wasn't quite sure what to do with it. It was pretty to look at that's for sure. This all reminded him that he should probably make a permanent den for himself sometime where he could store all his stuff. Having random stash piles in every cave wasn't nearly as efficient because all his possessions were scattered, requiring longer travel time should he need something.
Anyway. After the long flight back to Leo, Callum felt considerably hungry. But despite the variety of prey available to choose from, he thought he could try to find something different. After all, he didn't have much to do, so it wouldn't hurt to try and find something new for once. Leo was such a big cave, surely there could be something else to eat.

The Lessers of the caves were certainly singularly... singular. They didn't really evolve, not over thousands of years; they didn't change, or experience genetic drift, or interbreed. That wasn't chance: it's how they'd been designed. Admittedly, those actual designs were all over the place, what with the effects of magic: within a species, coat length, antlers, color and even limb count was... flexible.

So, why were there no Tunnel Runners with Jungle Deer markings; or Frostbiters with Lurker camouflage..?

Even here, Callum could find a strange, perhaps creativity-sparking example of this: down in the lava-flow caverns beneath Leo, a single Lava Rat had killed--and was eating--its close cousin, a Red Rat. Why didn't they interbreed?

Surely a Red Rat with a nice, shiny collection--or a Lava rat, really, with some sparkling metal--could have, at some point, attracted a mate from the opposite species? But here they were, killing and eating one another, savagely defined.

So, they didn't really do it on their own--but maybe, with the right setting, they could? And what were the limits?

It'd take a creative and dedicated mind to figure this much out. And in the meantime, there were free rats to eat.

@Callum

As Callum pondered over the idea of hybrid lessers, he found that the grumbling in his stomach had only increased. It was clear that it would take much more effort to find one suitable for his tastes than just searching. Perhaps for now he could catch a few things to satisfy his appetite before he continued on that train of thought. The dragon left the open bay, charting course for the heated caverns that lay below. For once in his life he really wasn't that picky about what he ate. He would just catch the first easy prey he sees. The gradual change in temperature from a pleasant cool breeze to searing heat took a bit to adjust to, and was still bearable. After all, he was both dragon and fish. The best of both worlds.
It was here that he saw there were several mice and rats running about. The lava rats were a bit harder to both catch and eat, so he settled on catching a red mouse.

His gaze fixed upon a particular mouse that had stumbled awfully close. A paw slammed down on its tail, and it nearly slipped away. Nearly. Callum didn't have time to savor the taste, he bit it in the neck and swiftly swallowed it whole. The grumbling subsided for now. It was barely enough food for a full meal, not even close, but it was enough for now.
The dragon watched a lava rat savagely kill a red mouse. If only there was a way to make them friendly with each other, so that they could be friendly. He sat and pondered this question. It was simply the way of life, for lessers to hunt each other, and for greaters too, like himself, to hunt them too. The task seemed rather impossible, but surely there could be some way to stop such a thing from happening.

Although he couldn't think of a way to resolve the central conflict, he figured that he could resolve the initial step first while he thought about it. Callum collected some loose rocks that had been littered around the area, and got to work on stacking them into a circular enclosure which could be used to contain the lessers that he put in later. Unfortunately, it was incredibly hard to find any that could fit into each other in the right shape. Putting a rounded edge against a curved edge meant that they could escape, not to mention it was hard to find stable rocks that didn't tip over with the slightest force.

But Callum could not give up. Now that the seed of an idea had been planted in his mind, he was set on achieving his goal. The stones that were greatly misshapen were dropped from a height so that they shattered and could be used to fill in the gaps. It was slow and arduous work, but at least it worked. Slowly but surely, the enclosure started to come together. He then had an idea to use the dust produced from the shattered stones to fill in the gaps further. He packed the rocks together as tightly as he could, even using his claws and teeth the shape the edges so that they fit incredibly tightly.
Soon, the enclosure was done. At least it was strong enough to hold some mice inside for sure. The walls were tall enough that they couldn't jump out, and smooth enough they couldn't climb out either. He had to say, he was rather proud of his work.

Now that the first step was done, Callum returned to the question of how he could keep them content and prevent them from killing each other. His loud stone dropping had scared away most of the prey in the area, meaning he would have to wander rather far to find some later. Hopefully some would return now that the area had quieted. Although he was starting to get rather hungry again, he was lost in his thoughts, ignoring the grumbling once again coming from his stomach.
And then it came. Aha! He had an idea. The reason that the lava rat had killed the red mouse in the first place was because it was hungry. Just like he himself was hungry! The only reason he killed was to satisfy this hunger, but if he provided them with plenty of food to eat there would be no reason for them to kill. Yes, that could work!

He noted that red mice seemed to always eat plants, but there weren't really any in this area. And that lava rats. Well. They ate red mice apparently. But it didn't feel right forcing another red mouse watching its kin be eaten. Perhaps he could find another prey and feed that to the lava rat. Plants were abundant around the bay so that would not be too big of a problem. He wondered if the rats would eat fish. There was plenty of fish in the bay too, so he could pick up some plants and fish all in one trip.
It didn't take too long to uproot a couple plants and snatch a fish near the surface of the water while he was at it. And with that, he carried these things back underground and placed them in the enclosure. Now all he needed was the subjects of the experiment.

He did not have to travel as far of a distance as he expected he would have to, and soon came upon a red mouse. Callum was not one for sneaky ambushes. It took only a few moments before he pounced, catching it by the tail then bending down to pick it up gently in his jaws. His stomach grumbled. It took a lot of willpower to stop himself from gulping it down right there. The mouse struggled, nearly managing to squirm out of his grip, but it was not enough. He carried it over to the enclosure and placed it inside. Now for the next one.

Catching a lava rat, however, would be a considerably harder task. They were much larger than red mice, not to mention much more dangerous. He reminded himself not to underestimate the rats like he had underestimated the meadow deer. Perhaps they would even put up a fight, and it would not be easy to capture one and make sure not to accidently kill one. Maybe he should target the tail and not aim for any other more sensitive parts.