How does Fluttershy know what an angel is? Religion doesn't really seem to be a thing in My Little Pony.

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The idea of an angel doesn't have to be associated with kind of practice of faith.

Fluttershy and her friends live in a universe with magic, which is a metaphysical force. And that means there is the possibility, at least for contact with metaphysical beings. Creature that ponies in older times could have called "angels" and "demons" for various reasons.

It depends, in part, what those terms actually mean in pony culture.

That is an in universe explanation, or a possible explanation anyway.

The actual reason just has to do with the writers using the best words they had for the moment. Like we all do.

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could have called "angles"

There are many good angles in MLP. You may even find a cute angle here and there.

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I fixed my little goof. I will be laughing about this later, but not right now.

7738998
Well if we want to approach this question within the show's boundaries, an angel could be viewed as one who has departed from that world and accomplishes tasks that are deemed honorable from the afterlife. It's basically the ghost equivalent of calling a person (pony) a 'hero'.

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--Sweetie Belle

7739032 Wow, they even dared to show an obtuse angle! What if a child sees this? Think of the children! They hate math.

7738998
He's actually an angel. A biblically accurate one. Be not afraid.

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Start Wars has Hell and Angle

https://images.app.goo.gl/Ti1o4fizQykmUccg7

https://images.app.goo.gl/cYKPPEJDGdyiDBgn8

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Hell and Angle

I bet the angle of Hell is obtuse.

Edit: and Lord of the Ring Orcs know what a Menu is

https://images.app.goo.gl/kza1U8nyogyFMY576

https://images.app.goo.gl/Z3hUrNa3htNDttKt5

7738998 In Season 4, Applejack mentioned "Sin" and "Heaven". Ponies probably do have some kind of religion, especially since it's implied that they worship Celestia like a deity.

7739183 Also this: