In the books, the dwarf joust at King Joffrey’s wedding (between four dwarfs) in season four is actually between two dwarf jousters, a brother and sister, one impersonating Robb Stark and one impersonating Stannis Baratheon in "A Feast for Crows." Those two jousters are actually quite important for Tyrion’s future, as he runs into the sister later in the books in "A Dance with Dragons." It was Littlefinger's idea and Joffrey was bored by the suggestion at first, until Littlefinger told him it would embarrass his uncle Tyrion.
The sister is furious at Tyrion because her beloved brother had been killed for being mistaken for Tyrion after Cersei offered a lordship to any man who brought her Tyrion’s head. The two end up becoming almost friends and are sold as slaves together as a jousting act (with Jorah Mormont, who doesn't have greyscale, remember?) in "A Dance With Dragons."
Also, the jousters ride a dog and a pig in the books, and in the show they don’t ride anything and just have animals made into their costumes so it appears they’re riding something.