for simplicities sake, I'll just go with the ones living with me right now in my tiny apartment.
this is fish-

he goes "blublubOMGPERSON BE AFRAID OF ME I'M BIG SEE HOW BIG I AM" then I walk away and let him think he won.
he was randomly abandoned in my old apartment when everyone was moving out, I never did find out who left him.
this is rat, or houdini, or muffin, or STOP BITING ME YOU BRAT

he's like the horses at the zoo, he can't tell the difference between french fries and fingers. plus, he's smart enough to be terrified when someone who's basically 200' tall picks him up and starts cuddling. I love him to bits though; I wish I could pick up a buddy for him, but I'm reluctant to spend any more money at the pet store he came from, plus it's kind of an endless cycle of buying rats to keep each other company.
at least he has Bun for company.

they share the same area, and like to harass each other. she's skitterish but she's grown accustomed enough to me to let me pet her.
I love the patterns on her fur, she looks goth. if she were a lop (floppy ears, like Vinnie's bunny) she'd be emo.