The Hidebehind is a silver-haired spectre that lives in North America and resembles a tall, skinny bear. It is named for its ability to contort itself in order to hide behind objects of all shapes and sizes. It can also become invisible. The Hidebehind preys upon humanoids, killing them by disembowelment, and only a witch or wizard is likely to be able to survive when faced with one (Pm, FB).
The first Hidebehinds were the offspring of a stowaway ghoul and a trafficked Demiguise, and were born on the ship where their parents mated. They got loose when the ship landed in Massachusetts (FB).
Isolt Sayre saved the Pukwudgie William from being killed by a Hidebehind. Later, Mr. and Mrs. Boot mistook the Hidebehind for a Boggart. The Boggart banishing spell was ineffective against the creature. Isolt and William were unable to save Mr. and Mrs. Boot, but did manage to defeat the Hidebehind and save their two boys, Chadwick and Webster (Pm).
Some experts suspect that the Hidebehind goes after humans because its Demiguise parent had such a cruel owner in Phineas Fletcher (FB).
Commentary
Etymology
The creature's name in Muggle folklore comes from the fact that it was said to be able to attack a person, then hide behind a tree before anyone else could turn around and see it. 
Pensieve (Comments)
Tags: deadly forest hidden hiding silver tall US Magic Remembrall