Tarzan the Terrible was serialized in Argosy All-Story in early1921 and appeared in book form soon after. I first read it when I was a kid, and again when I went on a Burroughs binge twenty years ago and read his complete works over a period of about six months.
In the land of Pal-Ul-Don, along with the usual complement of unique monsters and beasts, Tarzan meets and befriends two opposing humanoid races. One is white-skinned (the Ho-don), the other black and covered with long black hair (the Waz-don). Both races have long tails, which they employ as an extra limb. It's the sort of social dynamic later seen on many episodes of classic Star Trek. The Ho-don live in cities and palaces, while the Waz-don are relegated to the jungle. They're not exactly at war, but their co-existence is far from friendly.
To complicate matters, there are power plays at work, with the priesthood and other factions within the Ho-don itching to take over the throne. And to keep things lively, there are three romantic triangles. A Waz-don warrior is in love with a woman held captive by the Ho-don, an exiled Ho-don noble is hot for the Ho-don princess who's promised to sleazeball, and Tarzan's whole reason for being there is to search for Jane, who was abducted by Germans in an early book. More complications and abductions, ensue.
And to top it off, there's a mysterious white man (without a tail) on Tarzan's trail. That guy's identity remains a mystery until the very end.
Many of Burrough's books take jabs at religion, but I found this one surprisingly daring, especially for 1921. Learning that the Ho-don believe their god Jad-ben-Otho to be tail-free, Tarzan presents himself to the royal court as the son of that god. There are repeated references to him as "the son of god," and there's even a moment when Tarzan, hard-pressed by doubters, cries out, "Who dares believe that Jad-ben-Otho would forsake his son?" Gotta wonder how much flak ERB took for that one.