Complete Monster: Tirek is a surprisingly vile creature from a show that's supposed to be a little girl's show. A demon of a centaur, Tirek was the ruler of the Midnight Castle, and planned to send Dream Valley into a state of eternal chaos. To do so, he used a sack containing magical spells in it to essentially torture and transform young ponies into his hideous, winged dragons which he could then use to pull his Chariot of Darkness. He's also amazingly violent. This is shown whenever his henchman Scorpan failed him. He would threaten to have Spike's head cut off if he ever screwed up again, which makes it even more disturbing in that Spike was a child, and therefore it meant that he had no limits to who he had to hurt in order to get his way.
Contested Sequel: To a lot of fans either everything that came after these two specials, or just anything after Rescue at Midnight Castle, period.
Firefly. Spunky tomboy with guts and attitude? Check. Inspired Rainbow Dash? Check. Being voiced by Sandy Duncan also doesn't hurt.
The Moochick: this character is fondly remembered by many fans of the series as one of its finer points. Being voiced by Tony Randall probably helped.
Applejack, too, considering she was only in "Midnight Castle" but later got a song and a few generations later became one of the main characters of Friendship is Magic.note Of course, this was in part due to legal reasons. Applejack is apparently the only pony from G1 that Hasbro kept the trademark for.
Tirek is also surprisingly popular with the Friendship Is Magic fans. The creators eventually brought him back in said series.
Medley, Bow-Tie, Glory, Twilight, Moondancer, and Embernote One of the only G1 foals not to be a copy of an adult. all have garnered their place amongst fandom.
And of course Habbit, the Moochick's long-suffering rabbit assistant.
The Sea Ponies. They're especially popular with the Friendship is Magic fanbase, which is still waiting for them to show up in the series proper. They finally make an appearance in the feature film, with the voices of Kristen Chenoweth and Uzo Aduba!
Gannon-Banned: Some fans will insist that it's Rescue at Midnight Castle not Rescue from Midnight Castle, since it was given this name in it's later incarnation as a regular series episode. However, this is an aversion as it was originally called simply "My Little Pony" when it was first aired in 1984.
He Panned It, Now He Sucks: Expect anyone who dislikes Midnight Castle to get this treatment, even if they did have a well thought-out reason why.
Hype Backlash: A lot of fans hype these two specials up a lot as proof that not all of the G1 cartoons were terrible. Expect this to happen from time to time.
Tirek threatening to behead Spike, a baby, to punish Scorpan for his failure. If not then, the 'turn ponies into mindless monsters to pull his chariot' is probably the point where Tirek crossed this line.
Catrina almost crosses this when she attempts to kill a Bushwoolie and Baby Moondancer. Rep saves the former, and regarding the latter, he stops her and makes her change her ways just in time.
Nightmare Fuel: Tirek, everything from his voice to turning ponies into dragons, even leaving one out and forcing her to watch.
Sacred Cow: Midnight Castle is generally this for G1 fans. Catrina is this to a lesser extent.
Seinfeld Is Unfunny: To a lot of FiM fans. Though these specials were pretty edgy for My Little Pony, the current incarnation has incorporated aspects of these specials, particularly Nightmare Moon and a more powerful Tirek, leaving some G4 fans unimpressed.
Shipping: Pairings like Firefly and Medley, Bowtie and Applejack, Megan and Scorpan, Twilight and Moondancer, and of course Catrina and Rep have popped up in the rare fanfic.
Tirek was a veryEvil Overlord with intentions of ushering an age of eternal darkness and thus rule Ponyland and possibly Earth for eternity. To do it, he put the local enchanted creatures through painful transformations and constantly kicks his right hand goat-man, Scorpan around without relent and with threats of decapitating his only friend Spike. Scorpan himself was formerly a prince who had previously been enslaved by a previous conquest by this guy. He only got one half-hour to show his stuff, and he spent most of that time sitting on a throne.
Firefly, despite all her Wolverine Publicity during G1, never appeared again in the cartoons. It's an especially bizarre case because she kept being sold. However, every time she got re-released, they'd just re-run or re-release the one special she was in. She was sold like she was the Optimus Prime of the franchise even beyond the show's era, to the point of being on shelves alongside My Little Pony Tales, or even instead of MLPT, nearly a decade after her debut.
Tough Act to Follow: If you start watching the G1 cartoons starting with these, depending on how you view these specials, or even the franchise as a whole, you may or may not be less than impressed with anything that followed after.
True Art Is Angsty: Tends to be considered by some fans among the best things put out by the My Little Pony franchise because of its darkness.