First ... that's Evilbeeker, sir.
Second, Guenhwyvar is technically a magic item, though there is an argument out there if it is a real cat or not. Most argue it's like summoning a genie from a bottle. Bob's books refer to it as a "black panther that lives on the Astral Plane summoned to the Prime Material as a figurine of wonderous power", hence Guenhwyvar is not
native to the Underdark. Also, Guenhwyvar never had "armor"; so technically, not Guenhwyvar.
Lastly, and just a wild guess, but I'm thinking SSG doesn't have permission to call anything "Drizzt" or "Guenhwyvar"; hence, "Underdark cat". Salvatore's books are very "hands off" by copyright, so I'm sure that's a topic to be avoided by SSG.
I would have liked a spider. Plus, we already have panthers, so reskinning the old one to just have armor seems like an easy out.
Thanks for listening!!!
EB