Fenris is a creature that appears in Thor: Ragnarok. Based on the creature of the same name from Norse mythology, she is a giant wolf that served as Hela's war-mount during Asgard's age of expansionism eons ago. After Hela was banished to Hel by Odin, Fenris died and was laid to rest in the crypt beneath the Asgardian Palace.
When Hela returned to Asgard in 2017, she used the Eternal Flame to resurrect Fenris and the thousands of Berserkers that she had been buried with. Hela would use Fenris as a tool to intimidate the Asgardians into serving her and giving her information.
As Heimdall guided the surviving populace towards his observatory, he spotted Fenris and an army of Berserkers guarding it from a distance. Fenris charged towards the Asgardians before Valkyrie shot at her from the Commodore, but the gun had no effect on the giant wolf. At this point, Fenris resumed charging towards the Asgardians. Just as she was about to attack, Bruce Banner hit the bridge in front of her. Fenris paused and curiously sniffed at Banner's unmoving body to see if he was dead or alive. Deciding he was no threat, Fenris prepared to attack again, but was suddenly dragged back by her tail and thrown away.
Banner had survived his plummet from the Commodore and then transformed into the Hulk as Fenris was stepping over him. The giant wolf attempted to devour him, but only succeeded in tossing them both into the river below. Later, Fenris is seen leaping towards Hulk trying to devour him again, but Hulk keeps her massive jaws open. Eventually, she snatches Hulk in her jaws, throttles him around like a chew toy and dunks him in the water, where she bites painfully into his highly impervious skin with her razor sharp fangs. However, Hulk punches Fenris off, sending the giant wolf falling to her death down the waterfall and into the void below.