Hydia

Hydia is the main antagonist of My Little Pony: The Movie, and the 10-part episode The End of Flutter Valley, in the first generation of My Little Pony. Appearing in both a movie and a 10-part episode (where other episodes are 4 parts long at most), she is the longest running villain in the series, and one of the most evil.

Personality
Hydia is a genuinely malicious villain who had a stereotypical "wicked" demeneor and sought to utterly ruin the world around herself as well as abuse everything for no reason other than she could, she was also apparently part of a long legacy of witches and warlocks who glorified being wicked for no reason other than they could.

Hydia was especially abusive towards her two daughters Reeka & Draggle, to the point of punishing them for calling her "mommy" - a short-tempered crone and all-around nasty woman she lived within a volcano and cast many evil spells in her eternal crusade against all things "nice".

The End of Flutter Valley shows how evil Hydia can truly be as she allied with another villain to steal the Sunstone, which would literally cause Flutter Valley to die - allowing Hydia to take over the barren wasteland (which she preferred, detesting anything that resembled beauty or kindness).

My Little Pony: The Movie
In the past, Hydia unleashed a gargantuan slime monster called the Smooze, who destroyed the Grundles' kingdom.

Hydia decided to destroy Pony Land the same way. She had Reeka and Draggle bring her the ingredients to create another Smooze. They brought her all ingredients except the Flume, which neither wanted to get due to it being inside a vicious plant monster. Hydia threw the ingredients into the lava of the Volcano of Gloom to create the Smooze, who attacked Pony Land. Megan was able to defeat him with the Rainbow of Light, though he took the Rainbow in the process.

Reeka and Draggle were forced to admit to Hydia that they didn't get the Flume. Furious, Hydia sent them to get it. Once they obtained it (nearly dying in the process), Hydia fed it to the Smooze, returning him to power and making him stronger. When Megan and the ponies heard from the Moochick that the Flutter Ponies could stop the Smooze, she sent the giant spider Ahgg to stop them from reaching Flutter Valley, but they got by Ahgg by tickling him.

Hydia, Reeka, and Draggle rode a boat on the Smooze as he made his way to destroy Paradise Estate, but the Flutter Ponies arrived and threw the Smooze into the Volcano of Gloom, killing him and releasing the Rainbow of Light, which dropped the three witches in the volcano (though they survived, as they landed in the slime that was once the Smooze).

The End of Flutter Valley
In the 10-part episode, Hydia sought revenge for her defeat, so she sent Reeka and Draggle to ruin the Flutter Ponies' celebration. This failed due to Draggle reading the wrong spell, and the Flutter Pony Morning Glory decided not to go after the witches, mistakenly thinking they didn't mean any harm.

Hydia allied with Queen Bumble and her swarm of bees, who stole the Sun Stone from Flutter Valley. Without the Sun Stone, Flutter Valley began to wilt, and would become the dark, dank, and dreary environment that Hydia loves, so that Hydia could take it over. By sunset, Flutter Valley would be permanently destroyed.

The ponies and Spike went to Flutter Valley for help in defeating the Stonebacks, where Hydia attacked them with Reeka and Draggle. Gusty blew Hydia backwards before she had Ahgg trap them in a net of webs, saying she'd do something horrible to them after making them watch Flutter Valley die. Hydia told Draggle to cast a spell to turn the ponies purple, but it backfired and turned the witches purple instead.

After the witches reversed the spell, Draggle was tricked into releasing the ponies, but Hydia recaptured them with a magic wall, and Ahgg reimprisoned them in his web net. When Megan and the Flutter Ponies recovered the Sun Stone and made their way back to Flutter Valley, Hydia gave Ahgg a potion making him immune to tickling and sent him to stop them, but they dug a tunnel under him and reached Flutter Valley. The witches were forced to retreat as the Sun Stone was returned before sunset, saving Flutter Valley.

After Queen Bumble's redemption, Hydia, Reeka, and Draggle conjured a cloud to block the sun in a final attempt to destroy Flutter Valley, but the Flutter Ponies blew the cloud away, and, having learned from their past mistake, blew the witches away to parts unknown with Utter Flutter. They have not been seen since, and to prevent them from trying anything else ever again (having been spared twice before), they were presumably either imprisoned somehow or blown over a cliff (or possibly into the lava of their volcano).

Trivia

 * Hydia is the only main villain to appear in more than one storyline (The Movie and The End of Flutter Valley).
 * She is one of the most evil villains of G1, alongside Tirek, Squirk, Grogar, and Lavan.