Poptops are a player created unique monster type. They were submitted by Tomservo as an entry in the MonsterMonster contest, taking top prize which was addition of the monster to Starbound.[1]
They can be found in lush and jungle biomes. They will regularly whistle the Starbound theme song. There is a larger version of this monster as well, the Adult Poptop.
There is also an ingame plush toy object based on a Poptop.