Imo Mytical sims' special abilities don't need to be hardwired into the different types of mystical beings. Some would prefer a mammal looking sim with telepathy than a reptillian looking one. Id say, take the entire pet concept of choosing bushy tails/ears/mane color and transfer that to the sim's Mythical form in CAS mode.
Then, if you want special powers for these sims, have them learn it like the skilltrees in ROM. Only this time, please give them consequences so they dont feel OP like spellcasters. Have them leave furballs behind and crave hunting or something.
Edited for clarity.
