https://en.m.wikipedia.org/wiki/Fish

In a break from the long tradition of grouping all fish into a single class (‘‘Pisces’’), modern phylogenetics views fish as a paraphyletic group.

Paraphyly is a taxonomic term describing a grouping that consists of the grouping’s last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping (a clade) includes a common ancestor and all of its descendants.

This is in contrast to the class Mammalia which is a complete clade.

In other words, I could make up a branch of science called foobarthology that studies Jurassic raptors, whales, and the Rock Dove, but that doesn’t mean those things are related, or a ‘true’ scientific group of their own. It just means I put them together for some other reason, either cause it’s easier for the requirements of the job, or I wanted to, or many other reasons including historical.