The queen's main duty, as explained by the changeling, is to provide food for her subjects.
Since changelings feed on love, their main source of energy is provided by the queen’s love for her hive, which is in turned fueled by the hive’s love for the queen.
While she could fulfill that duty just fine, she was still just an itty-bitty changeling queen, and so required a regent to oversee most of the governmental work. However, this particular regent turned out to have higher ambitions…
It’s a Moon Roc!
According to the Big Book of Sciency Facts About Science, rocs are the most deadly of all the lunar creatures. They’re huge, incredibly powerful, and they eat just about everything, including little ponies. Especially little ponies.
It was nice knowing you, Woona.