Space Cases

While crew is collecting things from space they see an asteroid coming toward them. At first they're scared but they calm down when they realize it's a practical joke. They have accidentally brought onboard a cute little pet thing through the vacuum. The pranksters tell them it is a monster but the crew doesn't believe them. The pet has a big appetite and wants to eat everything onboard. It grows into a monster but the crew is able to flush it into space before it eats them.