None in the official rules.
The only source books other than DMG that include poisons are in Xanathar's Guide to Everything (poisoned needle trap and poisoned tempest), and the Players Handbook (Basic Poison Vial, and Poisoner's Kit).
It should be easy enough to homebrew, however.
The Shadow's Strength Drain is a melee weapon attack, with no save. The Intellect Devourer's Devour Intellect is a saving throw, followed by a roll contested by the players intelligence, and completely reduces there intelligence to zero on a failure. I don't think either of these are good templates to follow for a poison.
Some questions to ask:
- How readily available is this poison? Can it be purchased at the capital city magic shop only? or can it be picked up in any local adventuring store? How likely are the players opponents to have these poisons?
- Is the poison something your players can create themselves, same as a magic item? In that case, what "rarity" would you consider the poison in terms of how long it will take the play to create the item? does it require some special ingredient, such as "condensed Shadow" or "Intellect Devourer claws?"
- Is the poison stacking? Say you reduce by 1d6 on a successful hit, can you deal another 1d6 if applied again? in this case, what happens if the ability score is reduced to zero?
- How is the poison applied? Remember the list from the DMG includes poisons applied by ingestion, inhalation, injury, or contact, what would work best for this?
- How long does the poison last, and how is it cured?
With these questions in mind, you shouldn't have too much difficulty creating something your players can work with or against.