From Final Fantasy Wiki
Stingray is an enemy in Final Fantasy V, a rare encounter that can only be found at the sea area southeast of the Phoenix Tower. Note that it will only appear if you stay near the northern coast of the sea area, near where the Tower of Walse sank. It also appears in the lake near Carwen. It is another enemy with Fin, a Poison-inflicting attack, at its disposal.
Stingray's rare encounter rate is due to the valuable elements it carry. For one, it has the Blue Magic spell Mighty Guard, one of the more elusive Blue Magic spells (only obtainable by using the Beastmaster skill Control). The other is the rare drop Dragon's Whisker, a whip effective to use against dragons, and is valuable during the Shinryu and Neo Shinryu battles.