I generally hold onto my pets for a long time because I don't like change. Most of the time they're not going anywhere. I'd kill for some name changes though... Also this is by no means exclusive sometimes I couldn't figure out where I wanted certain pets or I might have missed some.
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Abbreviations NFT - Never For Trade PNFT - Probably Not For Trade PUFT - Possibly Up For Trade UFT - Up For Trade RUFT - Really Up For Trade