I had an Abominable Snowball on an old foster of mine that just kept gaining and losing levels and turning into soot. The more I zapped it, the more worried I became that it was going to vanish. I don't know where vanished Petpets go but I'm pretty sure it's a scary experience. So in the end, when I gave the pet to my brother (she hadn't yet been put UFA), Lunch the Abominable Snowball went with her.
Wow, sorry to hear about that. D: That's super bad luck. The only reason I got the petpet lab was for the avatar. There's so many petpets and colors that it's not really worth zapping for a specific one.