Yeah, I checked everything handy I have that uses that art, and none of them credit it with a specific source or artist. Probably because it was on just about everything - it’s kinda like asking “who drew the logo?” I imagine, just like with a logo, some studio was commissioned to come up with it, and RB then passed it around to whoever had a license to merchandise the stuff in the 80s.
You’ll notice with this artwork, it doesn’t look like, stylisticly, anything Rankin Bass or the animators put out (aside from the show itself, for example, the character sheets) and it doesn’t look like the work of anyone from Marvel, either. The characters have a bit of a different style to them, some details are different and…where the heck is WilyKit? :roll:
You may have them here at TCL too, but I’ve got some ‘art elements’ RB passed around for people to use, and it includes some of the characters in the artwork in question although by themselves and uncolored. Sadly, the letter RB sent with those images doesn’t explain who created the art either, just their rules about use of this promo art.
I’m sure someone already thought of it, but anyone think to ask someone like Tim Perkins about the art? I know nobody at Marvel drew it, but they used it on the annuals like many others who were licensed to crank out Thundercats merch. He may remember what the deal was with that art. Or maybe even ask that guy that runs the RB site now…Rick, uh, something lol
As I think it likely something RB had commissioned anyway - something uniform, along with the logo, that would tie all the stuff together. One look at a list of who had licenses to make what back in the day and you quickly realize a lot of it had that art, in full or in part, on it. None of which apparently credits anyone specific for it.
Well at least now I don’t feel so bad for not noting a credit with it either - apparently, it’s not so obviously unturned 
Good luck.
PS btw, David Lima did a wonderful restoration job digitally on that art, and made a sweet desktop out of it, here. Just in case someone here hadn’t seen it before 