From Wiktionary, the free dictionary
From some + pony, by analogy with somebody.
somepony
- (My Little Pony fandom slang) Somebody.
- Coordinate terms: everypony, anypony, nopony
2021, Vylet Pony, “ANTONYMPH”, in CUTIEMARKS (And The Things That Bind Us):I messaged somepony over Tumblr last night / She said "RAWR X3" so it's true love at first sight
