Grimace was originally portrayed as a character who stole milkshakes.
When Grimace first appeared in McDonald's advertising, he had four arms and was associated with taking milkshakes. The character was later redesigned into the large, purple, friendly figure most people recognize, with a simpler personality and appearance.
That redesign changed Grimace from a mischievous product thief into one of McDonald's most approachable mascots. His later identity focused less on a specific food and more on friendship, enthusiasm, and participation in the McDonaldland cast.
The Hamburglar is the character associated with stealing hamburgers, so the two are frequently mixed up. Grimace's historical role is especially notable because it shows how McDonald's adjusted its characters over time to make the brand more family-friendly and easier for audiences to understand.