Pterya (Wing in the PS3 version of Final Fantasy X HD Remaster) is the second of Isaaru's aeons that he summons in a battle with Yuna in the Via Purifico. She is the same aeon as Valefor and uses the same attacks. After Pterya has fallen, Isaaru saves the best for last: Spathi.
Since Valefor herself is not available, summoning Bahamut is the path to take for this battle. Mega Flare will deal a large amount of damage, possibly even breaking the damage limit, and Bahamut is tough enough to withstand Pterya's normal attacks to refill his Overdrive gauge.
Pterya is the Greek word for "wing". Likewise, Pterya's original name, "Tsubasa," is the Japanese word for "wing".