The Universal Monster Rules on poisons says
A creature with this ability can poison those it attacks. The effects of the poison, including its save, frequency, and cure, are included in the creature’s description. The saving throw to resist a poison is usually a Fort save (DC 10 + 1/2 the poisoning creature’s racial HD + the creature’s Con modifier; the exact DC is given in the creature’s descriptive text). Poisons can be removed through neutralize poison and similar effects.
Emphasis mine. That's pretty much where the standard poison DCs are introduced.
Note that in Dungeons and Dragons 3.5, the spell poison followed a similar pattern in determining its DC but substitutes Wisdom for Constitution; the Pathfinder spell poison instead uses the standard spell mechanics to determine the spell's saving throw DC but also uses Pathfinder's more complicated rules for poisons as afflictions.