These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: My Little Pony: Friendship Is Magic S1 E1 "Mare in the Moon"
During Pinkie's surprise party, one pony in the background for some reason had Fish Eyes. The fandom nicknamed her Derpy Hooves and the name stuck, with even Lauren Faust using the name on occasion.
The mint-green unicorn that happily waves to Twilight as she heads to Canterlot's library is later seen at the Summer Sun Celebration hopping up & down excitedly while staring at the camera and standing next to a pony that looks like Bon Bon from My Little Pony Tales. The fans dubbed the unicorn Lyra, she's become almost as popular as Derpy and these were only the first of her many Funny Background Events.
Hilarious in Hindsight: Twilight's observation that "All the ponies in this town are crazy!" turns out hilarious when taking into account that every Mane Character suffers a nervous breakdown at some point over the course of the season. It's made even funnier when The Hub uses the voice clip in commercials for the show; it's not false advertising!
Jerkass Woobie: Princess Luna/Nightmare Moon. Her life wasn't particularly happy.