If it's as bad as you say to stay closeted then just be open about it. Don't go rubbing it in peoples' faces, don't gather the family together to 'come out' -- it's not that big of a deal, just allow people to realise you like My Little Pony. If they ask why, tell them: it's a good show, there's a fandom full of interesting people. They won't necessarily like it, they might make a few jokes at your expense, but no one's going to disown you or change how they feel about you over pastel coloured horses so relax.
My suggestion: set it as your cellphone / tablet wallpaper, or 'accidentally' leave an MLP site open in the living room. This way it looks like they accidentally discovered it and starts the whole thing off as not a big deal.