**UPDATED INFORMATION** Apparently all the Level 48 Pets can be used to create the Hybrid Holiday Crowns Pets.
Okay, so the buzz around the game has all been about Wintertusk as well as the discovery of 2 new Hatchery Hybrids. Apparently, according to information from THE PET MASTER, Frank Howard told him at PAX that these new Hybrids are actually the new Holiday pets, and there are 6 or 7 of them Total. Considering there are 6 WC pets, I'd probably say there would be 6 total.
Unicorn + L48 Pet = Nightmare (Black horse, flaming hooves, mane, and tail)
Dragon + L48 Pet = Pet Egg (egg w/ Duck feet)
Imp + L48 Pet = ???
Piggle + L48 Pet = ???
Bloodbat + L48 Pet = ???
Firecat + L48 Pet = Fire Beetle? (Pyromancer Johnathon's info, but no Pict)
Nightmare is replacing Black Cat this Halloween, according to Pet Master, and Pet Egg should be available from the Easter vendor. One of these hybrids is supposed to be a 4th of July style pet, and rumors abound about a Patriotic Leprechaun, so I'm thinking Stormzilla or Hydra + Imp.