The Sbug is a small, unkillable Zesbisan scavenger insect that is usually found in quiet or abandoned locations. They are not really enemies and do no damage to you.
They only appear in Super Metroid
 Super Metroid
Sbugs are actually the first life forms you encounter in the game. They appear on the walls of the first Craterian cavern room just after landing your Gunship and they scatter in different locations as you approach them.
They are also located both in Kraids lair feeding on a dead Space Marine/bounty hunter and the unlit sections of the Wrecked Ship. Interestingly enough, the spot of the dead Marine is the only location they indefinitely appear in, although they scatter when Approched, they respawn in the same location when you reeneter the room.