They are not natural, that much is clear. Someone made these - possibly for "a laugh" - "oh, what would happen if..." - and then when they got pecked to death by them they got their answer. Maybe they were made in a wizard's lab, maybe a demon pushed malevolence into a predatory bird - who cares? They're here and they are mean.
Three-feet tall, mostly bird-like, skin-wings that unfold and unfurl and allow gliding. They run on double-jointed emu legs, and have have claws at their wingtips that help them to climb and grasp. Their head has simian and avian qualities, forward-facing eyes, short plumage that covers their torso as well.
And the grimhook. Damn. An eighteen inch razor beak ending in a four-inch downward hook overbite. Nestling between a two-inch double underbite. When it bites it tears flesh, the over- and under-bites scissoring together, the beak edge cutting. It's not just a predator, it's a killer.
When a Grimhook Jay steps out from bushes or glides down from tree-tops, a mocking "HAW HAW!" call echoing, you will flinch, but you will think you can take it. It's not that big. And it's just a bird with a sharp beak really. When a half dozen follow it, you will run. That's the only thing you probably can do.
AC: Leather. 2HD
Attack: Snaps at people with that evil tearing beak. 2+d4 damage for a hit. Successive hits may trigger a bleeding crisis - first hit just deals normal damage. After that a CON check must be made with any bite wound from a grimhook jay; failure means that at the end of the fight, people who fail a CON check are bleeding. They will lose 1HP every ten minutes unless wounds are bound or magical healing is used.
Notes: Grimhook Jays are pack hunters. Generally they run in packs of 12+d6 birds, with one pack leader (a 3HD bird with a jet black comb of stiff feathers on their head; deals 3+d4 damage from slightly bigger beak).
They're not very bright, except at harrying prey. They run quickly, can glide competently but stand out in their usual habitats (undergrowth, forests, jungles). They are spooked by fire or sudden loud noises. They will run away if more than half their numbers are killed or if the pack leader is slain.
The beaks of Grimhook Jays will fetch a fair price (75-100sp, LotFP standard). They're difficult to remove though without damaging them. A cracked beak will fetch 4d20 sp. If you have the option, you're better off taking the whole head with you and letting the buyer sort it out...