I had the idea to breed animals of different species to create a cross breed mount. Perhaps stats for the animals like a walrus has vow speed and high durability and can be bred with something with high speed and low durability to make something in the middle. A weight system? For an animal to fly it needs to have a equal to or greater than its weight number. And not pounds just like a measurement system from 1 to 50 Where as a mouse is 1 and a whale is 50.
Nice mechanic! I think you should just be able to designate a creature as a mount using an item of some kind. That way the breeding mechanic already being implemented could apply to all of your friendly creatures.
This is actually a very interesting idea. It could give the user some usage of a Random-monster generator, perhaps in lieu of what the developers show off every friday; with parameters taken from both parents. Even without stats or anything useful, it would be a truly brilliant touch to let the users create their -very- own creatures by breeding crossbreeds until it turns out how they want. A Turtle with a T-rex head a go-go baby. Let's just not think about how mechanics would merge with animals. It's science.