Though not seen, it is mentioned by Saix when he planned to send Xion to deal with it during her time unable to weild the Keyblade, and Roxas to slay a Bully Dog. Axel was later sent to "keep it out of trouble", falling on his behind running after it.
Roxas is sent to eliminate an unidentified giant Heartless that has surfaced in the desert and seems to be the source of Agrabah's mysterious sandstorms. When he arrives at Agrabah, he encounters this Heartless, but defeats it in no time.
The Antlion is a massive, seemingly mechanical Heartless with a round, egg-shaped body comprised of several rotating rings. Each of these rings are white with black and yellow highlights, and each is lined by multiple, rectangular pillars. Most of these pillars are steel blue, though two on the first ring are a patchwork coloration of gold and copper. The Antlion’s rear sports a grey turbine that it uses for locomotion. Its neck is black and gold, while its comparatively small head is roughly spherical and white. Its head sports glowing yellow eyes, a large, jagged mouth, and two short "horns". It also has two large, steel blue and gold mandibles on the sides of its head. Its Heartless emblem is between its eyes.