The Sceadugengan (Singular: Sceadugenga), or "shadow-goer", are fantastical beasts which are neither living nor dead, and which can shape-shift. They dwell in the forests of England. The first element of the name is from Old English sceaduwe (shadow) and the second elements is likely to be from Old English gong (a going or journey) or Old English gan (to go).
The Sceadugenga are mentioned in Beowulf and form part of a sub plot in The Saxon Stories series by Bernard Cornwell.
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This is an excerpt from this link: Viking Answer Lady

Like the draugr, Grendel is a creature of the night. He is deogol dædhata deorcum nihtum (l. 2775, "the hidden doer of hateful deeds in the dark night"), the scriðan sceadugenga (l. 703, "the gliding shadow-goer"), who moves through the landscape of shifting shadows and intermittent moonlight so characteristic of the undead: Sceaduhelma gesceapu scriðan cwoman wan under wolcnum (ll. 650-651a, "the shadow-cloaked shape comes gliding, black under the clouds.") Grendel conceals his actions beneath a mist, Ða com of more under mistleoþum Grendel gongan (ll. 710-711a, "Then he came from the moor, under hills of mist Grendel went"), and the intermittent moonlight suggested by the clouds and mist may be responsible for the gleam of light from Grendel's eyes: him of eagum stod ligge gelicost leoht un fæger (ll. 726b-727, "from his eyes came an ugly light, most like a flame").