After a mighty storm has passed, You discover a old waterlogged chest, sealed with chains and weighted with stones. You find drowned form of a young animal. Suddenly, the creature's eyes open you have acquired an unusual pet.
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)
Wild carnivorous beasts invade the character's camp. During the attack, the character finds that he has the innate ability to command wild beasts (not monsters) to do his will (so long as it does not seriously risk the animals' lives).