The Bulette, also known as the Landshark is a voracious burrowing predator that looks a little like a shark grew legs and started burrowing through the ground instead of swimming through the water. Stupid, mean and hungry, they are some of the most devastating predators in the messed up ecology that is Dungeons & Dragons, swiftly driving off or devouring anything that walks on land in the regions they lay waste to. Thus, they roam incessantly, constantly searching for the vibrations of prey.
Bulettes are one a small group of monsters inspired by a bundle of cheap Hong Kong "dinosaurs" (mostly bootleg knock models of Japanese kaiju from films like Godzilla and Ultraman) that Gary Gygax picked up at a local dime store; this makes it kin to the Rust Monster and Owlbear.
In the D&D world, bulettes are believed to be one of many species of horrors brought of mad wizards experimenting with fleshcrafting - specifically, they are suppposed to the the result of merging armadillos with snapping tortoises, and perhaps a dash of demon's ichor for extra meanness. In some editions, they are said to relish the flavor of halfling meat, but to find elf-flesh too disgusting to swallow and dwarf to be only marginally tastier than elves. It doesn't stop them from killing either race, mind you.
Bulettes were introduced in the very first issue of Dragon Magazine, and then went on to appear in the Monster Manual for every edition from Advanced Dungeons & Dragons 1e through to 5th edition. Originally, bulettes were drawn with a more shark-like visage, complete with prominent spinal fin, but from 3rd edition onwards became more smooth and rounded-looking, making the shark-like aspects less visible.