Though not seen, the Antlion was mentioned by Saïx when he was planing to send Xion to deal with the Emblem Heartless, while Roxas was sent to slay a Bully Dog at the Beast's Castle. Axel was then later sent to "keep it out of trouble", falling on his behind running after the Heartless.
Later on, Roxas was sent to eliminate an unidentified giant Heartless that has surfaced in the desert of Agrabah and was the possible source of Agrabah's mysterious sandstorms. When he arrives at Agrabah, he eventually encountered the Emblem Heartless, Antlion. Roxas confronted the Heartless, and was able to defeat the Heartless in no time.