Sluggo

Sluggo is a non-sapient creature whose outside appearance is virtually identical to that of an Earth slug. The names of its species and home planet have never been revealed, but it is known that it comes from a planet with an argon-rich atmosphere.

Sluggo is most notable for being the first contact with a previously unknown alien species made by the starship Enterprise (NX-01), which visited Sluggo's planet in 2151. Sluggo was brought onboard by Ensign Hoshi Sato, who wanted to keep it as a pet. In spite of Doctor Phlox's efforts to simulate its natural environment, the alien wasn't able to adapt and became sick and unwilling to eat. At the end, Hoshi decided to leave Sluggo on a planet similar to its homeworld, where it could live in relative safety.