Unicorns' magic comes from their hands.

The more intense the spell/the more energy the person is using will determine how much they glow.

The glow will, at most, go up to the shoulders (sometimes the face).

Dragons and Draconequuses work similarly. When they use more magic, their features (claws, horns etc.) begin to show.

The glow shouldn't take over the body. This only happens when control of magic is lost.