Antlions are Monsters found in the Desert biome, along with vultures. Antlions are a rare sight, as you will usually only see one every couple of visits to the Desert. They attack the player by spitting, or shooting blocks of sand out of their mouths. This being said, they are the first enemy to shoot actual material as a projectile. For an Antlion to spawn, you would need at least three blocks deep of sand, the same amount needed to grow a cactus. Upon death, Antlions may drop coins, and very rarely, Antlion Mandibles. Antlions may spawn on the side of a wall of sand. When this happens, they sometimes appear to be climbing up the sand until they reach the top (See image below).
Many players like to farm Antlions for their mandibles in order to make the Sandgun. A simple way to do this is just to walk through the Desert until you see one. But there are some some other ways to increase the chances of an Antliion spawning. One way is to make sure that the Desert is at least three blocks deep of sand. If it is less than that, then an Antlion will not spawn. You should also remove any ores or anything that might get in the way, because they only spawn on sand.