Whilst I was looking for black kats, I managed to spawn mewkat. I was so happy about that little fella showed I started to dance around it, and make haiku about it, but apparently it didn't fancy my sick moves nor poetic abilities, and despawned on it own, dropping nothing at all. I wasn't fast enough to make screenie, unfortunately.

As far as i remember rare mobs, like mewkat, shouldn't despawn on their own like that.