Artist:  
“Unicorn study this time!
 
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”
 
For anyone wondering, since people on Deviantart were confused: Top left: Illusion, Top middle and top right: Dove, Bottom left and bottom middle: Anthea, Bottom right: Claire