Well, at least in The Edition That Must Not Be Named, a standard creature is...just that. The "baseline," mathematically speaking, for a given challenge level. "Standard" is thus contrasted against "minion" (or "mook" in similar systems), "Elite," and "Solo." A minion(/mook) is much more fragile than a Standard, but still packs a punch, the weak "cannon fodder" type. An elite creature is meaningfully stronger than normal, drawing on the thematic trope of the "crack team" of better-trained, better-equipped soldiers, though it need not actually mean that in practice (e.g. imagine a swarm of hive-mind insects, with throwaway mindless drones as minions, led by a more powerful but harder-to-raise "centurion" bug or the like.) A solo is a creature meant to be a challenge to a whole party all on its own.
A "standard" creature in other contexts is usually intended to mean one where if you had a squad of just that creature and nothing else, and that squad was the same size as the PC party, then it would provide a relatively ordinary encounter. Probably not deadly, but definitely intended to not be a cakewalk either.