Stevie was a leaf-tailed gecko that formed part of the crown that King Julien wore.
King Julien, as an act of gratitude to Alex the lion for frightening away the fossa that preyed on the lemurs, gave his crown to Alex as a gift, deciding on a much larger crown fashioned from orange leaves. In place of pride on the crown was Stevie, who Julien was happy to add to his crown, and at Julien's command, Stevie danced on the crown.
Later, Alex and his friends were to return to New York in a broken-down plane that had crashed long ago in the forests. Julien and Maurice decided to go along with the group, placing Stevie in charge of the lemurs until such a time as he would return. The lemurs did not take to this change well, and Julien, pulling Stevie close, listened as the gecko licked into his ear, stating something to Julien's delight. Julien stated to the masses what Stevie said in a short issue of chatters and chirps: "let them eat cake", much to the lemurs' delight.
At that moment, Mort came running towards the plane, hoping to join the group as they headed to New York, but Julien, displeased by Mort, threw Stevie at him while his lemur guards tackled Mort, allowing Julien and the others time to board the plane without Mort catching up.
A small, orange, leaf-tailed gecko, Stevie was designated part of King Julien's crown, a position which Stevie was pleased by. Like all geckos of his kind, Stevie was orange with dark brown stripes, a cream underbelly, spotted digits, a flat tail with notches and yellow eyes, which he would lick with his tongue from time to time.
Behind the Scenes
Stevie was created especially by the filmmakers, who, after hearing actor Sacha Baron Cohen's improvisation of the line "it's got a gecko on it!", found the line so funny that they created Stevie so as to keep the line in the film. 
The words that Stevie speaks into Julien's ear in Madagascar 2 are "let them eat cake!" This is revealed in the film's DVD subtitles and is a reference to a quote famously mis-attributed to the French monarch Marie Antoinette.
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