I've just recently decided to start posting some of the MLP related art I've started making. However, the most recent one was deleted for being "Human Only". If I can just get some clarification as to how MLP related content, even though it's "Human Only" especially given the Equestria Girls side of things, why the post would be deleted for *that* ruling, that would be very cool.
The character in the drawings were human versions of Pacific Glow, a niche background character from an episode from the MLP. The post was no different to post 529486 where the character was colored as they would be in the EqG world

Humanized versions of characters are still human and images depicting only humans are not allowed under the current uploading guidelines. The post you are referencing is 6 years old, posted in 2014.
I'll quote the uploading guidelines here:

Any submission before 2015 has been grandfathered in and does not need to reflect our current or future guidelines. In fact, there is a substantial amount of old art that would be deleted under the current guidelines, the existence of those old submissions do not validate or invalidate anything on this list.

tosmilearts said:
If I can just get some clarification as to how MLP related content, even though it's "Human Only" especially given the Equestria Girls side of things, why the post would be deleted for *that* ruling, that would be very cool.

This site doesn't really care what series a character is from, or what form a character may normally have. What matters is if the image contains at least one visible non-human character (feral, anthro, or humanoid). Just as a solo picture of Ash Ketchum wouldn't be allowed due to being human_only, despite him being from Pokemon which contains a swath of otherwise relevant animal characters. Humans can be included in a picture, so long as there's a non-human character there too.

There's exceptions for comics or other multi-page sequences. So long as there are enough non-human characters throughout, having a few human-only pages scattered about here and there is fine so as to avoid gaps in the sequence or story.

tosmilearts said:
The character in the drawings were human versions of Pacific Glow, a niche background character from an episode from the MLP. The post was no different to post 529486 where the character was colored as they would be in the EqG world

If I were to guess, I would say the rules were a bit looser back then (6 years ago), and the post has been grandfathered in and allowed to stay, even though it would likely be deleted if it were posted today.