From the Super Mario Wiki
Venom is a barren, deserted planet in the Lylat System that starred in the Star Fox series, as well as Super Smash Bros. Melee. Much like Corneria, battles take place on the Great Fox, which is flying over the surface. Unlike Corneria, battles take place on the Great Fox's four wings.
The Star Fox Team will sometimes appear in Arwings and open fire on players, and on occasion, Star Wolf will as well. At one point in the stage, the Great Fox goes into a cave. While in the cave, if a player is sent flying diagonally, the player can hit the wall of the cave and survive.