Daddy Sad Heads are species of toxins which nest and reside in ponds. They have the ability to create a tear toxin that makes the host too sad to get rid of them. They nest in a big group underwater and when attached to a living thing, they release sad toxins.
In the episode "Water Park Prank," they were first seen attached to Princess B'Onangutan who was stuck in the middle of the pond on a tree trunk with the heads attached to her legs. They made her too sad to come out of the water. When Finn tries to rescue her, he carelessly walks into the water getting caught with the heads himself and becomes too extremely sad to get rid of them. Princess Bubblegum asks him to pretend he's a Jester and imagine the heads as bells and asks him to jiggle. He eventually shakes them off and shakes them off Princess B'Onangutan too.
They have the appearance of an old man's head with minor green strands of hair on it's scalp. They also have stubble and green hairs for a mustache. Their pink skin is wrinkly and they have very small eyes. The head is kept inside a jelly-like exterior sack, like the jelly substance in which frog eggs grow.