There is no story behind this, really. I just wanted to draw my Disturbed Nightmare Karrionyte towering over Sam as she brings him out of his field, because he's over 18 hands and she's a tiny (albeit tough) jockey. Karrion's usual rider, Nathan, has been working with Karrion since he was a foal, so Karrion behaves for him...more or less. But with other humans...well, expect malicious pranks and nasty surprises.
Here, he's licking the lead because his saliva rapidly rots things — his halter and rope are reinforced magically (hence the glowing) to protect them, but he still likes to test boundaries and make people nervous. Sam doesn't let him get away with any nonsense, and he knows it, but it's in his nature to push buttons. He just can't help it.