I wasn't sure if this should be in the news section, or in the other toys section, because this isn't a product by Hasbro or TakaraTomy, and it doesn't appear to be a licensed product either... as it emphasises that these toys that they make are "parodies", which can be a really grey area for getting away with copying ideas, as long as it is making fun of them (particularly in America).

So there is this series of toys called Hangrees, produced by an American company called MGA (they did the Bratz dolls)... and these Hangrees toys are like those dolls that you fill with water and they pee, except these ones are filled with a sludge mixture that comes out the backside like really bad diarrhoea.
They have a fair few different ones so far, with parody names, but the one to note here is a 3-pack that has an constipated Optimus Prime, called Pooptimus Prime.
(they look a lot like small Mighty Muggs - you move their legs to open up the "back passage" to let loose the sludge)

A youtube video of someone reviewing a couple of these toys, to show how they work.

And some photos of the Pooptimus figure being tested out, as it is currently available in America for about US$30 (also on Amazon).