Thanks for reporting this! I discovered this morning I actually can move them at least, last night I thought I couldn't even do that. But even so, they are taking up precious real estate.
I got one from the armored dragon the other day (the dragon we earned the first time we had to fight nagas.). I figured it was just a rare spawn, since I normally have a troll parked. Although, I have noticed the elves have started to ask for the naga drops.....maybe that's why they're spawning again.
Back on topic
My swamp kitty tried to drop a creep tooth this morning. They may earn permanent spots on the board if they keep that up!