My Little Pony: Friendship Is Magic fandom is  an animated television series produced by Hasbro
I've heard of it, I think the popularity is tied between the brand and quality writing and show design.
The series was tied in with the 2010 launch of dolls and play sets, and original programming for U.S. cable channel Discovery Family 
Seems like strategic planning on their part, Lauren Faust was also selected as the creative developer.
Under Hasbro's guidance, Faust developed the show to appeal to the target demographic of young girls and their parents
I agree, and think that she has made characters that challenge norms of typically "girly" images.
It also found an unlikely audience in a large group of adult Internet users in late 2010 and early 2011
Yes, I have heard that these fans are called brony or bronies.
to the show's characters, stories, animation style and influence of the show's propagation as an Internet meme. The fandom adopted the name brony (plural bronies), a portmanteau of "bro" and "pony". Though initially considered to propagate the humorous and ironic concept of people enjoying a show intended for young girls, the fandom has shown deeper appreciation for the show far beyond this concept and is considered part of a New Sincerity trend. Its technology-savvy members have created numerous works in writing, music
My Little Pony: Friendship Is Magic fandom
"" is an animated television series produced by Hasbro as part of the "My Little Pony" toy franchise, which is tied in with the 2010 relaunch of dolls and play sets and original programming for the American children's cable channel Discovery Family (formerly Hub Network). Lauren Faust was selected as the creative developer and executive producer for the show based on her previous animation experience with other animated shows such as Cartoon Network's "The Powerpuff Girls" and "Foster's