Frankly, I don't understand that it's like to be bullied for being a brony or for watching MLP. However, I do understand a few major motivations of bullies.
Bullies may have something wrong in their own lives, causing them to lash out at others to make themselves feel good, even for just a little while. Others are simply bored, and your reaction is entertainment for them. The last of these, from my understanding, are jealous or envious of some aspect of your life (which may even tie into the first category).
From my personal angle, I find bullies to be wretched creatures. I don't so much as regard them with contempt, but rather with pity and sympathy. I find that something you pity if far easier to forgive, once you understand them, and then, eventually, ignore entirely.
Another bit of personal advice is to own it, the fact that you can like/watch MLP. The confidence you build in the assurance that you can do as you please is powerful. Far too many people are coerced into dancing to the beat of another's drum. My only word of caution is that MLP shouldn't be your sole interest. From personal experience, having few interests makes it hard to carry a conversation past small talk (unless the stars align and you are speaking with other bronies).
Speaking of other bronies, see who else likes MLP. Hell, just find friends. They don't need to even like the show, they just need to be decent human beings (which, contrary to what the world around us looks like, do exist, don't worry). If you and your friend stick together, most bullies won't try to bug you- it's hard to antagonize for long when you're in the minority.