Yellow M&M's have been around since 1941, when M&M's were first produced. In 1995, when the blue M&M was introduced, M&M "spokescandies" were also introduced.
The yellow M&M was first voiced by John Goodman and later by J.K. Simmons. The yellow M&M, said to be happy and gullible, is the spokescandy for Peanut M&M's. His "likes" include pretty ladies and fluffy things (he had me until fluffy things). Interestingly, yellow is absent from the pretzel M&M packs. Apparently there is some sort of rift between yellow and the pretzel community.
Earlier this year in Las Vegas, my sister ran into the yellow M&M and knew she had a great Friendly Encounter opportunity. Ironically, Jean's husband despises peanuts.
Jean Parker and the Yellow M&M in Las Vegas, NV-March 2011.