'Spatterdashes' is the proper, albeit now archaic, term for spats. Circa 1680 it became (at least somewhat) common to replace full boots with a buttoned leather or fabric 'sleeve' covering the lower leg. These were much easier to doff and don, much easier to wash, and cheaper. They were worn by practical men, perhaps most particularly soldiers, although these were most useful to horsemen who were more worried about mud-splatter than the ergonomics of marching.
As with any fashion, it didn't take too long for the well-off to signal their status with clean white or ornamented spatterdashes, and to drift from practicality in other ways; notably, they became shorter, in two ways. Firstly, they became shorter in length, covering only the top of the shoe and the ankle, eventually becoming so attenuated that they did little more than cover the top of the shoe. Secondly, the name became severely truncated, with the first recorded use of the slangish 'spat' appearing in 1779.
Because we live in a soulless and aggressively boring timeline, spatterdashes are today generally known as gaiters. It should be noted that spatterdashes are different from the chaparejos ('chaps') worn by American cowboys, as chaparejos go all the way up the leg, rather than stopping at the knee.