"The first major enemy you'll encounter on this journey is the Titanic Ant. To sink it, you'll have to use both PSI attacks and regular Bash attacks."
EarthBound Player's Guide

Titanic Ant, known as Giant Ant in Japan, is the guardian of the first Your Sanctuary location in EarthBound, Giant Step. It resembles a large Black Antoid, and was probably altered by the power of Giant Step. It is accompanied in battle by two Black Antoids. 

The Gigantic Ant, a palette swap of the Titanic Ant, appears much later in the game.