The leading character is a Vietnam vet who is transported to a fantasy land. He gets a horse who he calls JEB Stuart. Other in the story is a beserker, a King who fights with his fists and a other person who has be transported from Earth.
Possibly "The Doomfarers of Coramonde" by Brian Daley? Some of the clues check out, but it's been so long since I read it I can't remember a lot of detail.
Just yesterday, Sergeant Gil MacDonald and his APC crew had been fending off an ambush in a Viet Nam jungle. In the middle of the firefight, some kind of magic spell had transported them to this Fantasy Land complete with flying dragons, wizards, crazy castles, and dispossessed princes. They would stay trapped here forever unless they could rescue the sorceress Gabrielle. Master magician, Amon, held her captive in his palace; and to reach her, Gil and his men would have to infiltrate Hell itself.
Google books has supplied the detail that, yes, he did name his horse Jeb Stuart. I'm calling it!
Gil was enticed into jumping Jeb Stuart through a flaming hoop,
There was a sequel called "Starfollowers of Coramonde", which I never read.