Grimace was the giant purple McDonald's mascot introduced in 1971.
Grimace first appeared as a much more mischievous character with four arms and a connection to milkshakes. McDonald's later simplified his design and repositioned him as a friendly, child-focused member of the McDonaldland cast.
The character became one of the most recognizable figures in McDonald's branding alongside Ronald McDonald, the Hamburglar and Mayor McCheese. His rounded purple appearance made him visually distinct from the food-themed characters around him.
Uncle O'Grimacey is a different character: he was presented as Grimace's Irish uncle and was associated specifically with the Shamrock Shake. The two names are easy to confuse, but Grimace is the primary long-running mascot.