Hostile encounter. Followers of a god are persecuting unbelievers, you had a direct or indirect interaction with them. (work with your DM to determine details)
Rebellion: As an adolescent you joined a rebellion against the local ruler, if successful you are outlawed, if it failed you escape, with only friends and family knowing your involvement. (Work with your DM to detirmine the details)
While foraging or hunting for food, you have an encounter with a trapped predatory beast. Instead of slaying it, you free it. The freed beast disappears. Later, you are trapped by a falling log. The same beast reappears, aiding and freeing you.
Monster. A monsterous enemy could be something as weak as a goblin to as powerful as a ancient dragon. They consist of monsterous races and creatures. They might have done battle with the creature, been robbed by it, wanting revenge, or hunting it.