In mythology what was the half horse fish called?

There is no widely recognized mythological creature that is specifically a half horse and half fish.

However, there are a few possibilities depending on how you interpret the "half horse" and "half fish" parts:

* Hippocampus: This creature, often depicted as a seahorse, has a horse-like head and neck but a fish-like body. It is associated with Poseidon (Neptune) in Greek mythology.

* Kelpie: While not strictly a "half horse," these shape-shifting water spirits from Celtic mythology are often depicted as horses with fish-like tails. They are known for luring people to their deaths in the water.

* Triton: Triton, a Greek sea god, is often depicted with a human upper body and a fish-like tail, but could also be represented with a horse-like body and fish-like tail.

Ultimately, there isn't a single mythological creature that perfectly fits the description of a "half horse fish." The closest might be the hippocampus, though it doesn't have the typical horse body.