I love everything about this! I'm pretty sure that J.K. stated that his patronus was a wolf, but he hated it so he always preferred to produce a non-corporeal patronus (which is the kind that you drew, where it doesn't assume a particular form). Very well done!
I love how you drew Lupin. But I can't fault your dementor. The flowing, tattered cloak seems so real. The detail is brilliant. I don't know whether you intended it, but I actually noticed a small skull shape near Harry's sleeve, which I really liked.
Fantastic picture. JK said that when a wizard/witch is in unrequitted love, their patronus assumes the shape of their love's patronus; and I recall early in the 6th book Harry noticing that Tonks' patronus was something akin to the canine family, although, as you said, it was unspecified.
Water bender. District 10. Daughter of Poseidon. Ravenclaw. Shadowhunter. Taurus. Daemon named Castor: Rabbit, Dormouse, Dik-dik or Sighthound. Daughter of the Spook. Princess of Narnia. Elementalist and follows the path of Geb. Mermaid. Student in the House of Night.