Happy Pets new Limited Edition "Shadow & Light Series" sports halos and horns, but unless you get a unicorn, these items are pet props that can be taken off and are not breedable traits.
Right now, the cheapest and hottest selling pets are the Sparkling Guardian Dog and the Hell Hound, which both go for 60 Facebook Credits (or exactly $6 USD). The Hell Hound is no Cujo, but it's clearly blood-splattered. And there's only three of the heavenly hounds left!
Both the Sparkling Angel Cat and Hellcat go for 70 FB Credits ($7 USD), while the Sparkling Guardian Unicorn and Raven Unicorn are both 80 FB Credits ($8 USD). Of them all, the Hellcat is the least popular, with still 401 left in the store.