So you want to make a robot, or a spirit companion, or some other sort of character? Well, this will take some conversation between you and staff to get the concept working, but it’s definitely possible, and very much allowed!
One good place to start is the Gods and Monsters book for M20; you can use the basic rules to make any of a number of companion-type characters. So if you want a talking cat, or a baby dragon, those rules will get you started.
Chimera are a more complex challenge; C20 Player’s Guide has a chapter on Chimera creation, both Lycian (free chimera) and companions. There are redes for Chimera in C20 and the Player’s Guide, but if you’re having trouble finding what you want, you can also refer to prior editions and we’ll talk about it.
I’m not setting house rules for these yet; the book rules seem workable for this purpose.