We need more discipline with the spankings. It a good way to deal with punishment can be more than spankings. Like soaping their mouths. Example of when it could be used.
Aj/rainbow for swearing (maybe twi too?)
Rarity, rainbow, for lying
Flutter shy—> peeved lol
Pinkie maybe for not shutting her trap sometimes.
Princess lie to protect others but I’m sure they use it to get out of things.

Also cornertime.
The easy way is to sit in the corner, but a more fun way is having their muzzle against the wall(since I don’t thing they could get their muzzle against the corner) and hold a bit(coin) there to make sure they stay in place.

Other punishments
Lines. And any of those in public, having the rail raised after a spanking, standing against town hall as a “corner”, holding a bar of soap in the mouth. All fun.

This was just a rant. Feel free to comment, if you wanna rp something related, that’s cool too

If soaping is going to be done in response to this, can at least someone show a little imagination in the setup? Like if the story involves Twilight and Spike, it's Spike who gets the chance to be the disciplinarian for a change and punish Twilight, possibly pointing out that as a Princess she should set an example for others, such as accepting the consequences of her actions when she does something wrong? Just because he's the youngest of the two, doesn't mean he should by default to be the one who gets in trouble.

Here's a good image to enjoy.

Also My Story Dolly Dash is soon going to have soap mouth usage in its next chapter.

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Foe some reason, I find soaping less acceptable than spanking. But I can run with unfair punishments too, so it's all good.

Cornertime is okay, but doesn't seem suitable for much except introspection.

having the rail raised after a spanking

What is this? Do you mean "tail"?

Chores can be good, especially messy ones like mucking out the barn.

And if the crime involves getting messy/covered in tree sap, then there are ways to make a bath feel like a punishment. Freezing cold water is good on it's own, whereas hot leaves the rump already raw in preparation for a follow-up paddling.