In Norse lore, Fenrir is one of the children of Loki, and he took the form of a giant wolf. Because of a prophecy, the gods knew that Fenrir was supposed to grow to great size (becoming larger than any wolf in the world, and larger than any of them). They also found out that he was supposed to kill Odin (the leader of the gods) during Ragnarök (the Norse end of the world scenario).
After finding these things out, the gods decided that they would take precautions. They restrained him so that he could not go anywhere, and he was left there for several years. When he eventually broke free from his restraints during Ragnarök, having already grown to a great size, he ended up attacking and killing Odin.
While what the gods had done would not have stopped Fenrir to grow larger than he was, what happened to Odin could have been avoided. If they had not restrained him when he had done nothing wrong, he may not have decided to kill Odin. Surely, he would have become angry enough by being restrained after having done nothing wrong that he would have wanted to take out his frustration on the person he blamed for what had happened.
Being a child of Loki, however, there are those who might say that the danger Fenrir posed to them was inevitable. Loki was technically a giant (as was Fenrir’s mother), and according to the lore, the giants were always trouble; they were prone to evil by their very nature. Since both of his parents were giants, and if they were truly the evil beings that the lore says that they were, it may have been possible that the danger he posed was inevitable.
His sister Hel is the goddess of the underworld, guarding over the souls of the dead (an important position to hold. If she had been trusted with such an important position by Odin, perhaps Loki’s children might not be as dangerous or evil as some stories paint them as being. As such, it is possible that the danger that came from Fenrir was more nurture than nature.
If another example was needed to prove that it was not necessarily Fenrir’s nature to be evil, Sleipnir might be what we need.
At one point, the gods wanted a wall to be built around their home, and they enlisted a dwarf and his stallion to get the job done. When the wall was almost finished, and it looked like the dwarf was going to get it done under the time limit (which would mean that the gods would have to give him treasures that they had no intention in giving him), Odin insisted that Loki make sure the dwarf did not finish in time. To make sure it happened, he changed himself into a mare and got the attention of the dwarf’s stallion, who left his work and followed after Loki.
The next time that Loki was seen, he was leading Sleipnir, an eight legged horse with magical abilities that he gave to Odin to be his steed.
None of the stories indicate that the horse was any danger to anyone, and he was immensely beneficial to Odin (helping him to travel between the worlds more quickly than he might have otherwise been able to do).
Since Sleipnir had never been treated as though he were a danger, no matter how he may have behaved, it was possible that there was no danger from Loki’s children. Some people might argue that since Sleipnir only had one giant for a parent, he might not have been as likely to be prone to evil as those children whose both parents were giants.
Whichever the case may be (if it was in Fenrir’s nature to be a danger, or if the danger was created by the way he was treated by the gods), Odin’s death is ensured when Ragnarök happens, and it will happen in the jaws of Fenrir.