ToME: the Tales of Maj'Eyal

Going along Old Forest with my Adventurer with Predator, I tagged a Treant fairly early as my prey to soften up Wrathroot. However when I found the bugger, he turned out to not be a Treant. You see, Treants are Immoble/Plant, but Wrathroot is a Giant/Treant. It appears that he's the only boss like this, at least in the West. Bill's a proper Troll, the Withering Thing is a canine, even the Weirdling Beast is an Eldritch Horror.

are you sure those aren't venus flowers?

Treants aren't Immobile either though.

Then perhaps the bug is that they aren't Giant/Treant like Wrathroot is.