Nibbler appears mostly on Fox's Futurama. He is a Nibblonian that was found on the collapsing planet, Vergon 6. We know him as Nibbler, but his true name would take the average human eternities to pronounce. He is actually from planet Nibblonia, not Vergon 6; a world with several dimensions of existence incomprehensible to the human mind.
Upon finding Nibbler, Leela and crew had captured two of every animal on Vergon 6. They stored Nibbler (who is relatively small) in with the other animals, hoping he would survive. When the last animal was captured, the crew opened the door to find the room empty, except for Nibbler. They placed the other animal next to him, and he consumed them all. His stomach has some mysterious ability to compact the animals as he eats much and even poops "dark matter".
Dark matter is an extremely dense substance, 'each pound of which weighs over 10,000 pounds'. This poop is used to fuel Planet Express's space ship.
Nibbler also has a range of other abilities. He is really an intelligent being who is capable of speech and operating complex machinery, like his space ship. He can cause memory loss by emitting a bright flash from his third eye. This causes a strange sensation, and appears to make the victim lose any memory Nibbler chooses.
Nibbler was the one who pushed Fry into the cryogenic chamber. He did this because Fry was the only person in the universe who could withstand the Brainspawn, because of his lack of the Delta Brain Wave. Fry travels back in time to stop Nibbler from pushing him into the chamber. Nibbler convinces Fry to push himself into the Future. Afterwards, Nibbler helps Fry to gain the love of Leela, by giving him a beautiful flower.