The secret aeon obtained from Baaj Temple in Final Fantasy X is Anima.
Anima is an optional aeon that can be recruited after the party gains access to the airship and returns to the submerged Baaj Temple. To open the Chamber of the Fayth, players must have collected the hidden treasures revealed by Destruction Spheres in the Cloisters of Trials. The process makes Anima a reward for thorough exploration rather than part of Yuna's ordinary pilgrimage route.
Anima's story is closely connected to Seymour Guado. Her fayth is Seymour's mother, who sacrificed herself to become an aeon in the hope that her son could defeat Sin. Seymour instead abandoned that plan and kept her sealed at Baaj Temple. Yuna can later claim Anima and use her devastating Pain and Oblivion attacks.
Anima is often confused with Bahamut because both are powerful aeons in Final Fantasy X. Bahamut is one of Yuna's standard story summons, while Anima is an optional secret aeon. The Magus Sisters and Yojimbo are also optional, but they are obtained through different side quests.