Shrek the Third
The Shrek Mascot made his first appearance in Shrek the Third. He was an actor in Prince Charming's play, where he played the role of Shrek. Charming had to slay him but it appears that he was only there for practicing because in the actual performance, Shrek himself played the role of the monster.
Shrek Forever After
The Shrek Mascot makes a cameo appearance when several fake Shrek's were showing up at Rumpelstiltskin's palace for the "Deal of a Lifetime". However, like all the others, he is not accepted as the real Shrek.
Ironically Shrek replaced the Shrek Mascot both times: