Lantean sea creature
It is not known whether this creature is a mammal, though it exhibits traits of Earth's whales, including a heightened curiosity and the ability to vocalize. Its hearing threshold makes it able to detect the high-pitched frequencies used by Puddle Jumper distress beacons. The animal can swim to significant depths well beyond 1,000 meters beneath the surface.
Grace Under Pressure - Dr. McKay owes his life to an enormous sea creature which keeps him company while Colonel Sheppard's Puddle Jumper tracks their position.
Echoes - The planet's whale population signals a warning to Atlantis of an impending natural disaster -- a burst of deadly radiation from the sun.