[Hall of Famer]

Well yeah the script is set up in a way that breeding between different classes is disallowed. The breeding class defines what species can interbreed.

Let's say you have three species: Lion, Tiger and Eagle. These are all different types of adoptables, but Lion and Tiger belong to the same breeding class Mammal and thus can breed their offsprings. The offsprings can take the form of a lion or a tiger(sorry, no possibility for liger yet). The Eagle, on the other hand, belong the a different breeding class Bird, and as a consequence it is safe to assume it cannot breed with mammals(Lion or Tiger).

Breeding of completely new creature(as in the above case, a liger) is not available in the current script. It will be done once a breeding system overhaul is completed, but right now we have much more important tasks to accomplish. I cant give a timeline for when this will be done, you can always attempt to code your own breeding system and raise questions if you encounter any issues.