This background pony first appeared as an accidental animation glitch with a cross-eyed expression and goofy smile; she has since become both a mascot for the fandom and a recognized character by Hasbro.
Derpy Hooves
100
Don't let his dull, docile expression fool you! Pinkie's pet alligator called this actually has a habit of biting anything near him; good thing the little guy is toothless!
Gummy
100
After secretly creating a potion out of Heart's Desire, Appleboom contracts this mysterious illness which causes her to gain multiple cutie marks and perform all their respective talents without stopping.
Cutie Pox
100
Since Ponyville was originally founded by Earth Ponies without the ability to magically change the seasons, this annual tradition was created to manually cleanup the mess left behind by winter and usher in spring.
Winter Wrap Up
100
Pinkie's Lie/ Rainbow Factory/ Beyond Her Garden
WoodenToaster
200
Hasbro recently announced this background pony's official name as "Time Tuner," but fans still commonly call him this based off a character in a popular British Sci-Fi TV show.
Doctor Hooves
200
Don't let his white and fluffy appearance fool you! Unlike his owner Fluttershy, this little guy isn't afraid to express what's on his mind and put his foot down to get what he wants.
Angel
200
This magical plant technically doesn't poison it's victims, but rather plays practical jokes on it's hosts resulting in a variety of physical aliments including: unresponsive muscle control, body shrinkage, and voice impairment.
Poison Joke
200
During this annual celebration, ponies don a variety of costumes to party, play pranks, and give each other candy of which a percentage is then offered to a statue of Nightmare Moon giving thanks for sparing Ponyville that night. Funny how no one recognized her in the pilot episode...
Nightmare Night
200
Discord (Remix)/ Good ol' Days/ Sister Hate
The Living Tombstone
300
Despite only appearing in the show for under 20 seconds, she become an instant hit among fans for both her sophisticated demeanor and talent with classical music.
Octavia
300
Even Rarity has problems keeping her own prima donna pet cat in her place, but it should be no surprise that they both share an impeccable taste in fashion.
Opal
300
When a sleep deprived Applejack accidentally added the wrong ingredients to Pinkie's muffin recipe, she and scores of other ponies came down with this illness soon after eating them. Spike seemed unaffected by them though...
Food Poisoning
300
The largest trademark of this annual winter celebration is with a massive pageant in Canterlot that tells of how the three different tribes of ponies came together to found Equestria.
Hearth's Warming Eve
300
Friendship/ Spirit of Chaos/ Monster
Aviators
400
This background duo is often shown performing various activities together with one rarely appearing in scenes without the other close by in early episodes; this has since lead to the fan interpretation that they are actually a couple.
Lyra and Bon Bon
400
When writers wanted a pet that matched Applejack's hard-working personality, they came to agreed on a Border Collie named this because of the dog's natural herding abilities and fierce loyalty to their owners.
Winona
400
Fluttershy is supposedly suffering from this pony illness when Rainbow Dash attempts to recruit her in breaking the tornado speed record. Funny how a bucket of water cured those spots right up!
Pony Pox
400
Double Jeopony! Another tradition stemming from Ponyville's original founders and their inability to use magic, it's original purpose was to clear the trees of their leaves so the winter snowfall wouldn't damage them.
The Running of the Leaves
400
Spectrum/ Crepuscularity/ Balladshy
AcousticBrony
500
Double Jeopony! Despite appearing in other earlier episodes, this background character's popularity exploded after she was seen secretly drinking directly out of a punch bowl. This has lead to the fandom's comical portrayal of her as the local town drunk.
Berry Punch
500
When considering a pet for Celestia, writers eventually agreed on the idea of a pet phoenix named this because of the mythical bird's symbolism for rebirth, royalty, and the powers of the sun.
Philomena
500
Although it's unclear if only pegasus can contract this illness, it's shown to be highly contagious when at least seven members of Rainbow Dash's tornado team is incapacitated by it.
Feather Flu
500
Taking place on the longest day of the year, it's the annual celebration of the imprisonment of Nightmare Moon by Princess Celestia and restoring the sun over 1,000 years ago.
The Summer Sun Celebration
500
WoodenOverture/ Want It Need It/ Friendship Symphony No. 1 in D Major