Nice. Don't think I've encountered that either... not from what I can remember at least.

I like the rarity of them, but I'm not sure what I'd like to see from it. It's just a troll in the end - no resources I need from killing it, and the rarity of them would almost make me want to leave it alone for that sake. Should they leave a different kind of trophy? A 2*troll head?
Is there a kill-list in Valheim to see what creatures and tier of them you've killed?