Rescue at Midnight Castle

Rescue at Midnight Castle, also known as Rescue from Midnight Castle and released later as Fireflys Adventure, was a 1984 animated television special based on the Hasbro toy line, My Little Pony. It was released as a pilot of the proposed television series, and featured the voices of Sandy Duncan and Tony Randall.

At Midnight Castle, Tirek has the smallest pony, Ember, taken to the dungeons, and uses the Rainbow of Darkness to turn the other three ponies Scorpan captured into dragons. He tells Scorpan to bring him a fourth pony by midnight, threatening to behead his friend, the baby dragon Spike.Megan, Firefly, and the ponies Applejack, Twilight, and Bow Tie go to see the Moochick, who has magic powers and might know how to defeat Tirek. On the way, Applejack falls off a bridge into the river and Megan jumps after her, from which they are rescued by the Sea Ponies, who tell them to call on them if they need help again. Megan and the ponies find the Moochick, who says that to defeat Tirek they must take away his Rainbow of Darkness, which gives him his power. He also gives them the Rainbow of Light, which can help defeat Tirek. ........

Source: Wikipedia