Tetra 鱼为什么叫这个名字，它跟「四」有什么关系呢？

Tetra is no longer a taxonomic, phylogenetic term. It is short for Tetragonopterus, a genus name formerly applied to many of these fish, which is Greek for "square-finned" (literally, square-winged).

Additionally, tetras possess a long anal fin stretching from a position just posterior of the dorsal fin and ending on the ventral caudal peduncle, and a small, fleshy adipose fin located dorsally between the dorsal and caudal fins. This adipose fin represents the fourth unpaired fin on the fish (the four unpaired fins include the caudal fin, dorsal fin, anal fin, and adipose fin), lending to the name tetra, which is Greek for four.