Mari's favorite animal would probably be cats, I like to think that she owned a lot during the course of OLNF. Her spirit animal would be sloth though for... reasons.
Elias' favorite would be dogs, specifically dobermans and chihuahuas. I know that sounds weird but here me out on them protecting Elias from le bullies. I'm not sure what his spirit animal would be though, maybe mayfly if that counts.
Kels favorite is koi fish, halfway because his dead name is Koi (they still think it's cool) and also he thinks they're really pretty and has a big tank full of them. His spirit animal would definitely be cats though.