There are two versions of a Snozzwanger, one for kids and one for when they grow up to be rutting adults.
For the kiddies, the Snozzwangers are mythical beasts that roamed Loompaland in Roald Dahl's Willy Wonka series. The Snozzwangers would eat up the poor Oompa Loompas, so the orange folks started living in trees. Because the Snozzwangers and the other horrible creatures like Vernicious Knids were decimating the population, Willy Wonka invited the Oompa Loompas to come and live with him in his chocolate factory. The Oompa Loompas now work for him, grateful for the great chocolatier having saved them from extinction.
After the kids grow up, they can deal with the adult version of a Snozzwanger. These are prostitutes who only provide hand jobs or blow jobs for their clients. The term seems to be more common in the United Kingdom than in the Americas.