Knowing that Jake Scully is just John Carter in a squishy-liberal, politically-correct universe, I finally found the reference I was looking for: An uber-classic Frazetta illustration of Carter's son facing a Banth.
This is the very image I thought of when Jake first meets the Thanator, and when a Thanator bows its head for Neytiri to ride it.
To his credit, Cameron did say "Avatar" was an homage to all the scifi he'd ever read or seen.
Here's the conversations:
Fan: Were you influenced by...
Interviewer: I see elements of ...
Film Geek: Scene 28 at 14 minutes 32 seconds in looks just like...
Accounting Department: Will this thing make money?
Accounting Department: Then all is right with the world.
(One more tie-in: "Thanator" is also the name of the Jovian moon, Callisto, as used by its inhabitants in the Lin Carter homages to the Barsoomian and Amtorian potboilers.)