From the Super Mario Wiki
Crocodiles are real-life animals that have made appearances throughout the Mario series and its related series. The Kremlings are a race of advanced crocodiles, but typical crocodiles have made a few appearances throughout the Mario series as well. They mainly appeared in The Super Mario Bros. Super Show! episode "Crocodile Mario", where they were portrayed with voracious appetites, and inhabited Down Under Land.
In "Crocodile Mario", Kangaroo Koopa stole a Magic Statue from a town in Down Under Land that repelled crocodiles. Without the Magic Statue, the crocodiles were free to roam the town. The crocodiles came close to eating everyone there, almost eating the entire general store and the tree that everyone took refuge atop of in the process. Mario and Luigi eventually retrieved the Magic Statue and liberated the town from the crocodiles, who chose to pursue Kangaroo Koopa instead.
In Mario & Luigi: Bowser's Inside Story, a species of Fawfulized crocodiles called Crawfuls appeared in the game as enemies. They resemble bipedal crocodiles with large faces that have Fawful's signature glasses and toothy grin.