Well, you've got nothing to lose, right? Explain that the previous person never responded to your neomail, maybe they'll be nicer. I know I would return a pet too. In fact, I have! I used to adopt them as charity work and put them back when they had better stats.
That's rather mean, though, isn't it? Why ignore your request and then get rid of the pet?
I'd join their guild in a separate account, find out which members don't like them, and then slowly poison everyone against them, and eventually complain to the guild owner...
Did I just say that?
Edit: If you believe it is the same person (the hints are there in the ages, guilds, shops and such) then try to give them a subtle guilt trip. Strong enough that if they're the ones, they'll pick up on it, but subtle enough that anyone else won't. Maybe when they see you're still trying, they'll see you really want it.
And, you know, if Irony is working overtime, they may have lost it while transferring it. I'd hurry, because if this was the case (it could be) they might neomail the new owner and try to get it back too.