ventral fin

[Jacobanade]

either of a pair of fins attached to the pelvic girdle in fishes that help control the direction of movement; correspond to hind limbs of a land vertebrate

[ArthuraCig]

Fins located ventrally below the pectoral fins of a fish.

[Dave Paprocki]

A ventral fin is a type of fin located on the underside of a fish or marine mammal used for balance, steering, and propulsion. Ventral fins typically include the pelvic and anal fins.

[Urukdaf 0]

The posterior pair of fins under the body of fishes,
corresponding to the hind legs of terrestrial quadrupeds.