Pucks is a term for Faun.
From the section in the Carnival Row Role Playing Guide (co-written by the show-creator) describing the different races:
Fauns are digitigrade, with legs similar to horses. Their horns are generally curled but come in all shapes and sizes, largely depending on the region the faun is from. They are robust with strong bones, and their horns can serve as powerful natural weapons. Fauns are also quite hearty and resilient.
Central to faun identity is the nation of Puyan, from which their mysterious religion originates. Natives to the cultural center of Puyan are referred to as Puyoc, which is where the derogatory term “puck” stems from.
Of course, out-of-universe Puck is probably a reference to the mythological character Puck, and/or the Shakespeare character based on him, often portrayed as being a faun-like character with horns of some variety or the other, however as Carnival Row takes place in a world that is not quite ours, it had a separate source for the name.