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That is like asking a Star Wars fan why they would want to play a Jedi/Sith in a roleplay, or a fan of Star Trek why they would want to play a Starfleet Officer/Crew Member in a roleplay, or even why of fan of Warrior Cats would want to play a cat in a roleplay. It's Fun
Fans of the show often enjoy playing characters that would fit in the world or setting of a roleplay based on a series or franchise, which in the case of My Little Pony means playing a pony character (Or another race seen on the show). In MLP:FiM in particular there are no humans, so playing as such wouldn't really be fitting. Edited: Thomaslee on 30th Jan, 2016 - 4:06pm
The same argument could be made for Dungeons & dragons. " dwarves/elves/half-orcs? Its just a question nothing meant by asking it. What's the fun in roleplaying a dwarf/elf/half-orc that talks and acts as human? Why not just have a regular human character?" I am not trying to insult you, but help you understand why. Sometimes being something other than vanilla human can be fun. Just because a race does human-y stuff, or acts human-ish, doesn't mean they can just be replaced by humans. Humans can't kick/punch boulders into the distance, fly, or cast spells from a horn on their heads. And some people feel that roleplaying should be about being something other than yourself. I think ponies fit the bill. Sorry if this seems rude, it is not intended to be, but I sometimes come across that way without meaning to.
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I understand what you guys are saying I guess for me I cannot relate to playing a pony. The examples you used with elves and dwarves yeah they are different but they are still humanoid so I guess there are similarities. Maybe its because I've only seen animals as by the way in any game but in pony game you're giving them front stage.