A Mantu is a clothing item for sale at the Toy Barrel. It only appears in the episode "Toy Store of Doom." It is possibly a tutu for men; although, SpongeBob could have made up the term "Mantu." According to SpongeBob, they were on sale and he managed to get the last one.
A Mantu is different than a tutu, as a Mantu has "extra support."
It's unknown what happened to SpongeBob's Mantu after the episode "Toy Store of Doom."