Or creating a mutated-creature design template could cost some essence, and training a Bearataur or what have you, would use up all your mana for the turn, or something. And would greatly deplete your supply of x creature or human, based upon accidental deaths and such (high mana cost at least) ... say for instance I wanted to fuse lizards or snakes with my soldiers (or regular citizens) in order to make lizard-people or snake-people.
To make 1 lizardperson, probably 10 humans would die, to to experimentation and accidental deaths. Meanwhile it would probably cost 10-20% of my mana (something decent but not too expensive, as its a medium sized humanoid), and maybe 5-10 essence for the creature template. No further essence cost would be used, and I could make as many lizzie-people as I wanted based on ... I suppose a nearby forest or swamp with plenty of high quality lizards, 10 people per one lizzie-person, and a good bit of mana. Eventually I could get about 50 lizardmen roaming around, at the cost of many people of course, and then train them as a unit, since they are so expensive, I would probably train them as the highest-form of elite/longest to train unit currently available, but yea.
Of course, the population cost would be too high to have a good number of these creatures moving about ... and say to make a bear-ataur it requires 5 horses and 5 bears, or something, or 6 horses and 2 bears, just whatever sort-a thing. Not really sure how we can determine what sort of animals we have access too, but if there are local populations of various wildlife (or if its a terrain based thing that can be opened by some sort of "creature editor" page, and while u cannot see them they are a part of the terrain value, or something) ... although number of people, and number of horses, will of course be present, so at the least we could mutate some centaurs, but having some lizardmen archers for higher dexterity, or a different type of lizardman heavy-infantry for increased natural defense and strength (and fatigue perhaps) .... in the creature editor, I think you should be able to add bonus stats to the unit, on top of what that unit would normally recieve.
Although I understand that having such things graphically represented would require more unit models, so I won't be sad if they don't get in ... although having some tribal lizardmen scattered here and there, in swamps or caves (n possibly deserts) to interact with.