Unicorn

“It wouldn't be a unicorn without a horn. That's what the word means! Uni, for one. And then corn for, you know, horn. One-horned."
"Yeah, but they're supposed to be all peaceful and nice. Why would a unicorn need a horn? What's it do with it?"
"I dunno. For shooting off magic and stuff."
"Or they use it to maul things."
-Roshani Chokshi, Aru Shah and the End of Time

Wandering the wide Hargellish plain, unicorns are spirits both feared and revered. Quick, strong, and hardy—unicorns are often associated with travel, movement, and freedom. They are able to switch between humanoid form, equine form, and innumerable forms in-between.

General Features

Age. Unicorns can live for thousands of years. They reach maturity around 100 years old.
Size. Your size is medium, yet you are taller than most humanoids.
Size. Your base speed if 40ft.
Illusion Form. While you are a unicorn, you can appear more-or-less human⁠—though you are limited to only one human form, which you must choose at character creation. You can also appear in any of the transitional forms between your unicorn and human forms.
Goring Rush. Immediately after you use the Dash action on your turn and move at least as far as your speed, you can make one melee attack with your horn as a bonus action.
Horn. Your horn is a natural melee weapon, with which you’re proficient. If you hit with your horn, the target takes piercing damage equal to 1d6 + your Strength modifier.
Equine Build. You count as one size larger when determining your carrying capacity and the weight you can push or drag.

In addition, any climb that requires hands and feet is especially difficult for you because of your hooves. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

Finally, a Medium or smaller creature can ride on your equine back if you allow it. In such a situation, you continue to act independently, not as a controlled mount.