Probably Gamzee? Kro's more likely to adopt people; Porrim can barely take care of herself; GHB would just laugh and/or commit animal cruelty; Poriva lives with her dad and again, Kro is more likely to adopt a person than a pet; and Kankri would just tut and say that it's terrible that there aren't enough shelters providing care and adoption services to the poor things.

ailean for sure and he has canonically despite being allergic to cats (it was a litter of kittens)

That's hard to say. A lot of my muses already have pets and none of them were picked off the street.

I might have to go with Red though, a scenario that comes up is an AU version of him where he attended a petstuck fighting ring and bought their bait troll off them so he could save her from getting chewed to pieces, and then he actually just kept her. It was an impulse save, and while money exchanged hands, it was definitely an accidental rescue adoption.