(Had to say it once here, but as I don't often come here this is the only place on this wiki I'll mention it)
When the human ponies transform, they get actual pony ears, Twilight, Rainbow, and Fluttershy get actual wings, but instead of actual tails they get hair extensions that look like the tails of their pony counterparts. This includes Daydream Shimmer's transformation.
The only two transformations that come with tails are Demon Sunset and Midnight Sparkle (though she became a separate entity and is now the third character to officially die in MLP: FiM, first two being King Sombra and Arimaspi, but that's irrelevant).
Why is it only the evil transformations have tails? Is it because they consider tails to be a demon thing? If so, why, how does tails' connection to the devil trump their connection to ponies (and this is the show where characters do the most things with their tails, I've taken count, possible exception of Pokemon which got its 927th episode this December 17th)
Is it because of the Tailed Beasts in Naruto? (I heard someone say Naruto started the trend of shows coming on only once a week, but I haven't been able to back that up)
Is it because Mario uses a tail technique in Super Mario Bros. 3, which contains numerous references to hell?
Or is it just because the devil has a tail? Explaining why there's no horns for Sunset, Twilight, and Rarity either. Except Demon Sunset didn't have a horn, and Daydream Shimmer did.
Or, did one of Hasbro's monkeys just screw up, with a smarter crew member designing the dark forms? This would fit because Applejack and Rainbow Dash, as well as the actual book, call them "tails" even though they're not. So, was it supposed to be actual tails but someone screwed the design up?
That could be more likely, because if Hasbro thought tails were a symbol of the devil, then they'd never have allowed the kindest and cutest pony to use a major tail technique at least 11 times in Season 6. (though her first three voluntary uses of her tail are corrupted by Discord or as Flutterbat)
Yes, the devil has a tail. But so do ponies. So do horses. So do cats and dogs and birds and monkeys and foxes and fish and chameleons and mice and almost every creature on this planet except humans, apes, frogs, and a few others I can't think of offhand. But even humans have tails as embryos before they get absorbed.
Anypony who's heard me mention this on MLP Wiki and is a little tired of it, try to just bear with me and take the question seriously. This is the only time I'll say this here