Sid and Elvis are a pair of monkeys, the Jungle King's most loyal servants, who would do anything it takes to stop the kids reaching the temple. They even make some challenges harder, especially the Dark Swamp challenges, since they seem to like hanging around nearby the swamp.
At the end of each show, the monkeys lounge around outside the temple of the Jungle King, and without fail Underwood tells them to "clear off". They obey him in a most obedient fashion. They follow the orders of the Jungle King, and they do lots of stuff to stop the jungle explorers, for example, they yank the ropes which control the baskets movement upwards while people jump (or swing) for them. They always disobey the Jungle Guide's commands.
Sid and Elvis have changed colour throughout the series of Jungle Run. In Dominic Wood's series, Sid was Red and Elvis was Blue. In Chris Jarvis' series, Sid changed from Red to Yellow while Elvis changed from Blue to Orange. In Michael Underwood's series, Sid changed to Black while Elvis remained Orange.
Unfortunately, at the end of the series, they were both put down due to the pair contracting rabies, after Sid had bitten Elvis in a fierce fight at the season's finale. This left a somber tone for the rest of the episode, with the Jungle King sitting sadly in his throne, pondering how life could've been different.