The bullfrog is a live-action frog who appears in the episode "Rodeo Daze."
The bullfrog is dark green and lime green with tan leopard spots on his back. The frog also has black eyes. Like the other land animals in the show, he's live-action, not animated.
Role in episode
When SpongeBob and the other Bikini Bottomites come to rescue Sandy from the rodeo, they find that she is riding the bullfrog. Once SpongeBob greets her, this causes Sandy to fall off of the frog, causing her to complain that she lost the rodeo.
Afterward, Sandy warns SpongeBob that the bullfrog is coming right towards him. The frog first attacks the bubble in which the Bikini Bottomites are in, essentially popping it. Sandy then realizes that the bullfrog now has SpongeBob and lassos the latter directly to her safely, causing the crowd to cheer.