Their main special power apart from flying, they can take on the appearances of other creatures or objects, as well as their abilities, and voices.
In A Canterlot Wedding - Part 2, they originally served as the secondary antagonists and invaded Canterlot under Queen Chrysalis' command, but they are eventually defeated.
In To Where and Back Again" (Part 1), during a nightmare that night, Princess Luna alerts Starlight that the changelings have replaced herself and all of Equestria's protectors, including Twilight and her friends, in their plan to conquer Equestria; Thorax also approaches to warn Starlight and Trixie of his kind's occupation of the Crystal Empire.
In To Where and Back Again" (Part 2), Encouraged by Starlight, Thorax willingly gives Chrysalis his love, causing a powerful burst of magic that metamorphoses him into a new form. His transformation inspires the other changelings to do the same, destroying the throne and freeing all of Starlight's captured friends. Rejecting Starlight's offer for redemption, Chrysalis vows revenge and flees. Thorax becomes the hive's new leader while Starlight returns to her village's festival with her friends.
In To Change a Changeling, Starlight and Trixie are attacked by Thorax's unreformed brother, Pharynx, who refuses to adopt the hive's new, peaceful ways. The other changelings feel uneasy around Pharynx and elect that he be banished from the hive for his ceaseless bullying. Starlight and Trixie agree, but when they confront Thorax over the issue, he is reluctant because Pharynx would always defend him from being teased by the other changelings. Starlight is inspired to lure a maulwurf—a giant bear-mole creature—to the hive, expecting Pharynx to come to Thorax's rescue in an act of brotherly love, only to find that Pharynx has already left of his own volition. Starlight, Trixie, and Thorax find him fending off the maulwurf to protect the hive, proving that he still cares for the changelings' safety. After working together to defeat the monster, Pharynx is accepted by the other changelings and transforms.