There are 4 unique codex entries in the Species section on Tython. They are faction specific.
Flesh Raiders is obtained from speaking with Padawan Avitla during "Early Lessons" mission as a Jedi Consular or Jedi Knight player or killing a Fleshraider Shaman. It can also be obtained from killing Bloodskin, Cha'nagh, Flesh Raider Berserker, Flesh Raider Bloodguard, Flesh Raider Boneguard, Flesh Raider Chief Shaman, Flesh Raider Chieftain, Flesh Raider Deathcaller, Flesh Raider High Shaman, Flesh Raider Jailer, Flesh Raider Mangler, Flesh Raider Slavemaster, Flesh Raider War Leader, Grand Chieftain Keshk, Hulking Flesh Raider, or Mar'gavrok on Tython.
Codex type: Species | Faction: Republic | Class: All
Associated Achievement: None
Togruta is obtained from interacting with a Clickable Datapad.
Codex type: Species | Faction: Both | Class: All
Associated Achievement: None
Twi'lek is obtained from interacting with a Clickable Plaque.
Codex type: Species | Faction: Republic | Class: All
Associated Achievement: None
Zabrak is obtained from interacting with the Ancient Computer during "[HEROIC 2+] The Chamber of Speech" mission. Zabrak is unobtainable for Smuggler and Trooper characters.
Codex type: Species | Faction: Republic | Class: All
Associated Achievement: None