What do you call a mythical horse that lives under the sea or part of your brain?

There isn't a specific, widely-recognized term for a mythical horse that lives under the sea or part of the brain. Here are some options you could consider, playing with the concepts of sea and brain:

Sea-Based:

* Hippocampus: This is the actual name of a sea creature, but it is also a part of the brain, adding a link to your second idea.

* Sea-Steed: A straightforward combination of "sea" and "steed."

* Oceanic Charger: A more epic name, evoking speed and power.

* Abyssal Courser: A name that hints at the deep sea's mystery.

Brain-Based:

* Mind-Charger: A name emphasizing the horse's mental nature.

* Thought-Steed: A more poetic way to describe the horse as a mental companion.

* Inner-Rider: A name suggesting a horse that embodies one's inner self.

* Cognito-Steed: Combining "cognition" and "steed" for a more technical feel.

You can also be creative and invent your own name!

Ultimately, the best name for your mythical horse will depend on the specific characteristics and role you envision for it in your story.