it has been shown before that animals will put bones of there dead on. it is shown in Pokemon (cuebone) and how to train your dragon (bonerush), as well as tepical mythology. so what if Deerper puts on a deer skull and it makes him so frightening that Wendy gets scared of him
“Okay dude I know you’re out here.” Wendy moved a stray branch out of her way as she walked through the foliage of the woods. “What did you want to show me?” It was almost noon and the shade of the trees provided a nice retreat from the heat. She sniffed once more only catching scent of some flowers. “Dipper, don’t make me come after you!” A small smile played on her lips, their usual games consisted of Cops and robbers, hide and seek, or tag when it was just the two of them. But Dipper had told her that there was something he wanted to show her before running off into the woods. She stopped and leaned casually against a tree, closing her eyes and taking in deep breaths of the fresh air.
There was a snap of a twig to her right past the bushel of small bushes, behind an old log. Her eyes remained closed.
“If you thought you could sneak up on me then you were wrong.” There was no response, only a small thump on the other side of the clearing. “ Oh-kay, if you’re trying to scare me it won’t work either.” She just noticed how eerily quiet it had gotten; her ears twitched trying to pick up anymore small sounds.