Eels on Archer IV.
Eels on a Xyrillian ship.
Eels are cold-blooded aquatic vertebrates with serpentine bodies and a minimum of appendages. True eels are fish, although throughout the galaxy there are numerous species of animals referred to as eels.
Some eel-like creatures were seen swimming in a lake on Archer IV. (ENT: "Strange New World")
The Xindi-Reptilian biorifle weapon is powered by a lifeform resembling an eel. Its neural pathways modulate the weapon's power output. (ENT: "The Shipment")
The Xyrillian ship encountered by the Enterprise (NX-01) in 2151 had a tank in which eel-like creatures could be seen swimming in liquid through windows. (ENT: "Unexpected")
- It is unknown what function these creatures serve, although one could easily imagine them as decorative pets, such as the fish in Picard's office. Also, it is unknown what liquid they are swimming in, only that it can't be water, because we find out in the episode, that the Xyrillians have trouble producing water and that solid cubes with the consistancy of gelatin is the closest to water they can come up with.