THOOSA was a Haliad nymph loved by the god Poseidon. She was probably a goddess of dangerously swift currents, as her name suggests.

Thoosa may have been conceived as a mermaid-like woman with the serpentine-tail of a fish in place of legs, similar in appearance to her sisters Skylla and Ekhidna.

Homer, Odyssey 1. 68 ff (trans. Shewring) (Greek epic C8th B.C.) :
"Polyphemos, the Kyklops whose power is greatest among the Kyklopes race and whose ancestry is more than human; his mother was the nymphe Thoosa, child of Phorkys the lord of the barren sea, and she lay with Poseidon within her arching caverns."

Pseudo-Apollodorus, Bibliotheca E7. 4 (trans. Aldrich) (Greek mythographer C2nd A.D.) :
"A son of Poseidon and a nymphe named Thoose, an enormous man-eating wild man named Polyphemos, who had one eye in his forehead."

Theocritus, Idylls 11. 25 ff (trans. Rist) (Greek bucolic C3rd B.C.) :
"I [Polyphemos] fell in love with you [Galateia], maiden, the first time you came, with my mother, eager to cull the bluebells from our hillside: I was you guide."

Theocritus, Idylls 11. 62 ff :
"Only my [Polyphemos'] mother does me wrong, and it's her I blame. She's never said a single word on my behalf to you [Galateia], for all she sees me growing thin, day after day. I shall tell her that my head and both my feet are throbbing: so I'll be even, making her suffer, even as she makes me."

Sources:

• Homer, The Odyssey - Greek Epic C8th B.C.
• Apollodorus, The Library - Greek Mythography C2nd A.D.
• Theocritus Idylls - Greek Bucolic C3rd B.C.