Gustav is the son of Canoe Cop Sven.
Though this is never explicitly stated, Gustav makes reference to his father being the best friend of Commander Jackson. Given how chummy Sven the Canoe Cop and Commander Jackson became during It Came from Another World! and the resemblance between Gustav and Sven, it seems likely that Gustav is Sven's son and that Gustav was named after his father's partner in The Canoe Cops.
The Creature from Terror from Beneath the Earth is an alien - not a mutated bat.
It was never conclusively shown that The Creature was a mutated animal and an alien resembling The Creature is seen in the Bar Full of Aliens in Destination: Outer Space
