- Any mob (or their pet) with which you have less than friendly faction reputation (neutral, unfriendly, hostile and hated).
- A PvP flagged player (or their pet) of the opposite faction.
- Any player (or their pet) you are currently dueling.
- Critters. Those poor squirrels.
More generally, "enemy" may refer to the faction opposite the player's own (e.g. Horde for a night elf), or any faction opposed to the player's, regardless of attackable status.