There are two epic enemies on Tatooine. Plaguehorn does not have a codex entry, and the Trapjaw codex entry is not required for any achievements, but the three bosses are.
The Plaguehorn event world boss.
Codex type: Achievement: Epic Enemies
Codex location: Tatooine: Jundland
Codex coordinates: 1270, -1292
Faction: Both
Class: Bounty Hunter - Imperial Agent - Jedi Consular - Jedi Knight - Sith Inquisitor - Sith Warrior - Smuggler - Trooper
Associated Achievement: Plaguehorn Reek
Note: Although the achievement is not tied directly to killing Plaguehorn, the boss is the only source of the pet
Special Requirements: Only accessible during the Tatooine Rakghoul Resurgence event
The Trapjaw world boss.
Codex type: Achievement: Epic Enemies
Codex location: Tatooine: The Dune Sea
Codex coordinates: -1776, -870
Faction: Both
Class: Bounty Hunter - Imperial Agent - Jedi Consular - Jedi Knight - Sith Inquisitor - Sith Warrior - Smuggler - Trooper
Associated Achievement: It's a Trapjaw! | World Boss Ingredient Sampler | World Boss Ingredient Connoisseur
World Boss Ingredient Enthusiast