When Grendel is introduced in Beowulf, he is described as a thoroughly Christianised monster:
That fiend from Hell,/that grim spirit, was called Grendel . . . With fulsome monsters/this sorrowful man had stayed awhile,/Since the Shaper had condemned him/as Cain's kinsman. (ll. 101-107)
Because the Beowulf poet, most likely a monk working in a monastery (possibly in Mercia), needs to create a monster who is linked to the Christian world, rather than to the pagan world of Scandinavian or Germanic tribes, he gives Grendel a biblical pedigree--a descendant of Cain, who is cursed by God because he murders his brother, Abel. Aside from explaining Grendel's biblical origin, the poet also creates a monster who is essentially in the form of a man, in Grendel's case, a giant, but still recognizable as a man. Later, when Beowulf and Grendel fight at Heorot, Beowulf mortally wounds Grendel by tearing off his arm.
At the same time that we are presented with Grendel as a kinsman of Cain, however, the poet also clearly links Grendel to his Scandinavian and Germanic relatives. Grendel, perhaps once a man, evolves into a monster because he, like Cain, has spent so much time among monsters:
. . . for the Measurer banished him/for his crime, away from humankind. Misbegotten creatures came to life then,/eotens and elves and orkneys,/as well as giants, who fought with God. (ll. 109-113)
Eotens are giants or some other type of unspecified monsters, and elves (ylfe) are, in Scandinavian and Germanic mythology, supernatural entities who are most often thought to be enemies of mankind or at least not necessarily friendly to mankind. Both Scandinavian and Germanic mythology includes creatures that the Beowulf poet describes in Old English as eotens and elves, which could be synonymous with ogres and trolls. In Old Norse, for example, the word for elves (ylfe in Old English) is alfar--clearly, both words have the same origin. It is likely that the word orkneys (in Old English, orcneas) refers to evil spirits or monsters in the form of whales (the killer whale, for example, is an orca). The Scandinavian people would have been thoroughly familiar with whales; the Germanic people less so.
In sum, then, while Grendel seems to be derived from supernatural beings in both Scandinavian and Germanic mythology, the Beowulf poet, because he is writing a poem for a Christian audience, turns the original pagan monster into a biblical creature.