Goby
ventral fins.
it is modified into fins
Probably, dorsal, ventral and tail fins.
there a part of a fish that makes them swim
There are two sets of "paired" fins. The ones on each side of the fish behind the gills are the "Pectoral Fins" and the ones in front of the fishes vent on the underside of the fish are the "Ventral Fins". Sometimes these fins are wrongly refered to as "Pelvic Fins".
Rudd
The "ventrals" is simply another name for the fishes "ventral fins" These are the paired fins in front of the Vent (aka Anus).
a dorsal fin is the one (or more) located on the back of a fish or aquatic mammal - its the fin that you see when a shark is swimming just under the surface of the water. Ventral fins are the ones toward the rear of the body and help in steering the animal through the water
They are on the Ventral side, but you probably mean the Pelvic fins and possibly anal fin, depending on the fish species These links provided below may help you :)
The opposite term is 'ventral' - For example in fish (and dolphins given the category) the dorsal fins are on the top of the body, while the ventral fins are on the underside.
Female bettas have smaller ventral fins and an oviposter. Males have longer fins (expect plakats) and are usually a little larger.
An abdominal fin is one of a pair of posterior fins in fish - the pelvic fin and the ventral fin.