Drat. You said it three times. Now I must answer your question.
Honestly, I don't know. A jaguar is an American cat, not an African one - and putting one in the Black Kingdoms just seems wrong to me. I would substitute a Leopard Man (see Robert E. Howard's short story, Black Talons for an example of the Leopard Man Cult) or a were-leopard.
I wrote up a cult on the Leopard Men for S&P. I don't think it was ever published, though.
Anyway, I am not sure what the author intended there. I think using a jaguar creates the wrong atmosphere, though, as it calls to mind (well, to my mind) Central American cultures, not African. Maybe the author just liked the idea of a tiger-sized leopard in the Black Kingdoms, and instead of just describing it that way, used the word jaguar (which is basically an American tiger-sized leopard). Maybe the author just wanted it to be a mystery. I suspect the author just meant the word to refect a generic large jungle cat (in which case he should have used the word 'panther' since 'jaguar' is a very specific American cat).
If you do run it with actual jaguar men, be aware that jaguars have a rather unique method of killing their prey - they ram their long canine teeth through the skull and into the brain. I believe they are the only panther cats to do this. I could see 'jaguar men' doing this... it's a pretty cool image anyway. It's something a leopard-man wouldn't do.