You MUST:
credit me; in comments on dA, ON the photo outside of dA (bymelody.deviantart.com)
show me what you've made (in a link from another site is also ok)
aks me first if you want to sell it as a blend on any horse site for virtual money (howrse, hetpaardenspel, my horsez, horsetycoon etc)
You can:
use it for manipulations
use it for blending
use it for digital art
use it for line-arts (just be sure to tell everyone in your rules to credit back to me as well)
use it for coats on Howrse
use it as a reference for a drawing
use it for pre-cuts (just be sure to tell everyone in your rules to credit back to me as well)