The Giant Antlions look like ants and cockroaches mixed together than thrown into quicksand. On the World Map, they will only appear as a pool of sand. When you go up to this sand however, they will attack you. They have high defense and attack, so they are like a mini-boss if Salsa is underleveled. Then, use a can of Bug Spray, and they will be dead automatically. They don't give much EXP, but they are good if you are grinding.
One of the Giant Antlions captures a Save Frog. Defeating the Antlion saves the frog, so you can save your file.