I can't resist posting my reply to what has been said in this thread, as I fit the labels of My Little Pony collector/customizer/fan and furry pretty well.
In response to the OP:
Furry fandom reached its peak of popularity and was fully recognized as a fandom in the 1980s (source: http://en.wikifur.com/wiki/Furry_fandom
). It is largely based on anthropomorphic animals featured in sci fi comic series, although the influences of TV and Internet characters are not without merit. In this general (and I mean very general) definition, a furry is an anthropomorphic animal (typically walking, talking and acting human). As adults started furry fandom, there are adult themes with it (what fandom lacks this?? I implore someone to research this for psychology or something), which obviously divides the fandom rather than unite it. In addition, with the advent of the Internet and forums, lots of furries feel the need to express themselves a little too much and take things a little too far (I guess with animal-like qualities all sense of decorum is thrown out the proverbial window), as do all "geeks," "nerds," and "gamers." The term RAGEQUIT comes to mind.
My Little Pony has also been around since the 1980s. MLP began with a giant plastic horse with a brushable mane and tail, a winking eye and a poseable body called My Pretty Pony. After My Pretty Pony was a success, six smaller versions of it in pastel colors made of vinyl plastic were mass produced, thus beginning the franchise/empire dubbed My Little Pony. MLPs were wildly popular with little girls from the 80s until the mid 90s around the world, and several animated films, a few cartoons featuring the ponies, and tons of related merch, including comics from the UK, were made. In the cartoons, MLPs could talk like people, and often had issues that lots of little girls had (marketed directly to their target audience of course). Flash forward to 2003. MLP is relaunched, with an unsuccessful cartoon series yet tons of toys were sold, customized (I started in 05, fyi; http://enchantingcustoms.weebly.com/
), and marketed like hotcakes. Internet forums abounded with the MLP relaunch, and most communities are fairly peaceful though not without flame wars and RAGEQUITs of their own. People are people, fans are fans, and diehard fans tend to blow up more often than say the average schmoe. Anyway, back to MLP--in 2010, MLP was yet again relaunched, this time under the creative direction of Lauren Faust (source: http://en.wikipedia.org/wiki/My_Little_Pony
And this is when the brony culture was born.
Bronies are typically younger men, although a few women enjoy the FiM show (myself included). Bronies are, in my mind, a subset of MLP fans. Some MLP fans are, in turn, also furry fans, although the term is shunned. I think the main reason why furries are looked down upon is because of the negativity inherent in a lot of Internet talk from furries themselves (including the drama). In turn, bronies are just as bad--because of the stigma associated with the word bro. Bros, as the label implies, are "better than you are," fraternity inhabiting jerks (at least that's from my own experiences in SoCal, don't get me started on bros irl...). Being a bro irl is way better than being a nerd (and furries fall under that category all too well, I am afraid), so ostracizing and bullying follow with each term in kind.
That's why I think bronies react so harshly to furries, at least in some cases--they are still in the high school mindset of "bros are better than nerds." In addition, the more labels you place on someone/yourself, the more likely you are to be attacked/ridiculed/teased/bullied, so I think that may be another reason why bronies refuse to associate with the furry fandom.
As time marches on, it will be interesting to see if bronies hang in there when the G4 line ends, or if they will fizzle out with the show.