I dunno why specifically, I just feel like Poison should get one. Was thinking it could be called something like "Noxious Fumes," with the description being "this move chokes the target, stealing HP every turn."
Give it to Weezing, obviously. --- "Microsoft is not a helicopter." ~Gerald Ilukwe
Logically, it doesn't make much sense for poison to heal something that isn't poisoned. There is already plenty of overlap between Grass and Poison mechanically. Plus, Venusaur doesn't need two Leech Seed variants, but maybe Leech Seed deserves better distribution.
Also, Poison gets Black Sludge which is kind of the consolation prize for not having Leech Seed and being an unlovable type. Literally the only time Poison shines is in the hours of Team Rocket battles gen 1 and 2.