1990:
DRINK 'n' WET BABY PONIES
Oh no they didn't.
...Oh yes, I'm afraid they did.
1990 saw the first, and, thankfully, only year of peeing My Little Ponies. These four baby ponies were obviously inspired by the hugely successful Drink and Wet doll market (because who doesn't like a peeing toy, right?). They were sold in boxes and came with accessories, including a baby changer, two special nappies that showed patterns when wet (isn't the point to encourage them NOT to pee on their underwear?) as well as a baby bottle of a style unique to this set. Why was this? Well, the bottle was hollow and could be filled with water.
Each baby pony had her mouth open, and a plastic tube fitted from mouth to rear with the obvious effect of, yes, you guessed it, drinking and wetting!
Fun as I'm sure this was for lots of young children, it did create one issue for collectors and that was the risk of damp getting inside the pony. For the most part, the pipes held intact and most second hand DnW babies are not mouldy or horrible - but it is something to watch out for when trying to acquire this set.
The ponies had different names between the US and the UK. Again, Hasbro America favoured Recycled Name Syndrome, whilst Hasbro UK were more original in their choices of appelations.
The Drink n Wet Baby Ponies did appear in the My Little Pony comics. Each was usually involved in water-related stories, but, surprisingly (>.>) they never actually peed in the stories. Most common was getting splashed by river or lakewater or by jumping over puddles (a speciality for these critters).
Hasbro's sale premise clearly worked, because these are pretty common on the second hand market.