Taming an otter in Ark can be done by following these simple steps:
1. Knock out the otter with a bola or tranquilizer dart.
2. Place the unconscious otter in your inventory.
3. Force feed it Mejoberry, Narcoberry, or Biome-Specific Kibble (found under Consumables in your inventory).
4. Wait for the otter to wake up and follow you around like a baby dino!