Back Off! I'm the Shifter King!

Zeb, a shape-shifting King of the Solar System, arrives incognito to propose to his long-distance girlfriend Sadie, but she dumps him for Grav-Ball star Jack. He instead helps the President's daughter Kate escape a forced marriage to a rich jerk, Chad, by posing as her fiancé. Guarding Kate and her teacup dragon, their fake engagement turns into real love as he outshines Jack and Sadie. Learning that Chad is a Klathu, an evil bug-alien, he crashes the wedding, defeats it, and reveals his true identity.