Why do ponies use the word "everypony"?

[Acoustic Cloud 2,202]

The answer is simply English groupings for things.
When you describe a large group of people, you say, "a body"

When you describe a large group of horses, you say, "a herd"
So saying, everyherdy wouldn't sound very good, so being the pun filled writers they are.  It was apparent enough to change it to pony.  It rolls of the tongue, has a bit of a gag to it, and that's all there was to this rub really.

[Moon Rat 4,772]

The question then, is whether Twilight was referring to a group of buffalo and/or ponies. But I think you may be onto something that only groups are referred to in this way. 

 

 

well, if I remember correctly, when she says "everyone" or "everybody" she was referring to everyone there including the buffalo.

 

now whenever she says "everypony" she is referring to the group of ponies (even Spike because as far I know, he doesn't mind being referred to as a pony.)