I write MLP fan-fiction under the name Bad Horse. I'll skip the reasons why I write fan-fiction. There are a few reasons I chose to write ponyfiction as opposed to other types:

• There are only a few large, active fandoms: Harry Potter, Twilight, Avatar, Naruto, and My Little Pony.
• The ponyfiction websites are much better-designed than those for the other fandoms. There are no fanfic websites for any other fandom that have a ratings system, so it's too much work to find anything good to read. And I didn't want to just write and not be a member of the community. Even if I were arrogant enough to do that, it wouldn't work very well even from a completely Slytherin viewpoint. This is the most-important factor.
• You can write anything as ponyfic. If you want to write a story about an old woman whose children have left and who has refocused her attention on her flower garden, and a travelling salesman who figures this out and manipulates her love of flowers to sell her something, and you wanted to do it as a Harry Potter fanfic, you'd have to make her use spells to grow her garden, or something stupid like that. I can't even imagine how you'd do that as a Twilight or Naruto fanfic. But anything becomes a ponyfic when you make the characters ponies. This is probably why I like My Little Pony. Harry Potter is about the wizarding world. Avatar is about a world war. MLP is about people.