Delta Tail Betta

Deltas Tails are very similar to Halfmoons but have less than a 180 spread when flared. Deltas are far less than an Halfmoon. Deltas are differentiated from Veil Tails by the fact that if you drew a line from the nose to the tip of the caudal fin on a Delta there would be an equal amount of fin on either side of the line, whereas on a Veil Tail there would be little tail at the top, and the majority below. The Delta tail has a triangular-shaped caudal fin.

(Taken from here)