This is a Wendigo / Forest spirit / Leshen (from The Witcher) / Krampus costume set. Now with detachable horns and antlers!

It's remixed from other things (see remix sources).

UPDATE 2024-09:

I have added octagonal alignment slot and through-hole for easier antlers alignment and attaching. The 100% scale model should be fine to fit M12 nut inside the antlers base, and M12 bolt underneath the mask rear part. There're also antler “base” bumps on the rear side of skull to add thickness and make it easier to mount the antlers (should work both as glue-in or bolt-on).

You may also want to check out these articulated finger extensions by dome7801

UPDATE 2023-04:

As suggested in comments, I have added holes in the base of antlers to help with mounting them using screws. Also, simplified geometry, so that the updated models with mountholes have smaller filesizes.

UPDATE 2020-10:

You probably would want to check out the great articulated claw by JulienJeanroy. It's skinnable and you may want to create your own skin for this costume or use one provided by JulienJeanroy (although it doesn't quite fit the intended aesthetics, but it's really close to it).

Scale it properly!

For 57cm head circumference 103% scale up gives a snug fit, 104-105% should work better

For 58cm head circumference 107% scale up should work well

For 59cm head circumference 110% reported to fit well over a headmask

The mask can be printed in 3 parts if your printer has large enough build volume, or you can print in 6 parts (included in archive). Print with supports and probably with rafts (if you have problems with delaminating supports). Make sure to scale the mask right

Either way, you'll need to find a way to glue it, so find the proper glue for your plastic. There were no full mask model in the source thing, so I can't provide you with one.

Antlers are cut into parts to fit most printers. Print antler mount parts with bolt hole DOWN and use the PAUSE AT HEIGHT to insert M12 nut into the hexagonal cavity. Only use supports on the outsude, not the inside. For right/left antlers use mirror feature in your slicer. Use M12x40 bolts to attach antlers to the mask.

To properly size claws first print the Sizer part (included in archive) in different scales (start with 120% and increment up to 190%). Try printed sizers on your intermediate phalanges (second from the fingertips). It should fit snugly. When found proper scale for each of your fingers, print 4 ClawFinger and 1 ClawThumb per arm at desired scale. Claws are printed with supports and rafts.

All of these can be printed in draft settings, with maximum layer height and nozzle diameter.

Tips on completing the costume

For covering your body use full-body spandex costume. They're cheap and can be bought for any size. Black ones are quite versatile for many costumes (affiliated links ahead!):
http://ali.pub/56wqc4
http://ali.pub/56wqhy

If you don't have any time left for international shipping, use women's black pantyhoses or tights to cover your head. 2 layers of 20 DEN should be enough, but YMMV.
Don't forget about breathing! Although both lycra/spandex and nylon tights are quite breathable you may want to leave a little hole (fortified with heavy stitching to not let the fraying sides loose) so that you can breathe easier and drink at the party.