A huge venomous sea serpent native to the water world Argo, whose poison supplied the antidote that returned Kirk and Spock from water-breathers to air-breathers.
The sur-snake is a gigantic sea serpent found in the oceans of Argo, a world almost wholly drowned by seismic upheaval. Its venom is deadly.
In the animated episode The Ambergris Element, a sur-snake attacks the aqua-shuttle carrying Kirk and Spock. When the two officers are found, the native Aquans have altered them into water-breathers who cannot survive in air.
Records in the sunken ruins of the Aquans' ancestors reveal that the change can be reversed with a serum made from sur-snake venom. Kirk and Spock, helped by the young Aquan Rila, capture one of the creatures and extract the poison at great risk, and McCoy uses it to restore them.