Zubat Number 7 is one of Rusty's many Zubats that he has caught. This particular Zubat met an unfortunate end when Rusty tried to teach it Surf, even though Zubats cannot learn that move.
Zubat Number 7 looks like all other Zubats, except that he tends to sit down instead of flutter. After he drowns, his ghost is doomed to eternally drip with water.
Rusty tries to teach Zubat Number 7 how to surf so that he can bypass a troublesome tree in his path. Even though the Zubat clearly does not understand the command to surf, Rusty assumes that he understands and throws him in the water before jumping on top of the Zubat, drowning him after a moment of struggling and thrashing.
Zubat Number 7 eventually returns to haunt Rusty in the Pokemon Tower in Lavender Town. He and Rusty's other deceased pokemon try to kill him, but when Rusty's Grandpa Crispen intervenes, they stop to listen to him. They all flee when Rusty throws a pokeball at his grandfather and captures him.