actually your definition of "bending reality" is wrong. so sorry,
but what bending reality really means is to literally change whats unchangeable. not create.
since the term bending is to bend, or warp, ergo to change, discord could not have simply created screwball herself out of nothing, or build her out of matter himself atom by atom, since discords power is of course pure chaos, and obviously not that structured to be that precise..
rather, he most likely found some filly somewhere and used his power to change her into what she is.
the ol' kidnap a baby and raise her as your own method.
or perhaps created her using the common method, biology n' what not.
but since we don't know who screwball's mother is, the first theory might as well be true. until proven otherwise through the fictional scientific process of exposition given through narration of an episode in itself.
(so this is what it feels like to be Alex Trebech... ill take "A Nerds Diatribe" for 800.)