seaswine
A porpoise or dolphin.
archaic
The seaswine are almost always seen in troops, especially in their sexual season, which is in the month of August.
A ballan wrasse.
Where it comes from
From sea + swine. Compare sea hog. Compare also Middle English brunswyne (“porpoise”).
Adapted from the Wiktionary entry seaswine (CC BY-SA 4.0).