Interpretation 1, for those who like reading: Art based on the story Harmony's Wrath (incidentally, by me). Read to the whole story here, [link]
or rate & comment here [link]
Interpretation 2, for those who want an off-the-cuff random explanation: The main characters of the show had Alternian-style ancestors who looked almost exactly like them, and quadrupedalism is actually a larval stage of pony development.
Interpretation 2a, take it or leave it: Modern ponies lack a bipedal form due to an evolutionary process called neoteny [link]
where juvenile traits are retained into adulthood (in the real world this is seen in flightless birds and axlotl).
This explanation is not related to the linked fan fic.
Interpretation 3: HIGH FANTASY ANTHRO PONIES, WOOP
Interpretation 4: This is in fact a D&D game. From left to right, these are the party's Bard (with levels in Artificer), Fighter, Ranger, Wizard (prestige class: Eldritch Knight), Rogue and Druid.
Version without the iffy Bob Ross-style background: [link]
EDIT: 200 faves? Wow, that's... never happened to me before. Neat!