Good f***ing luck trying to get any work out of a horse wearing a gasmask, assuming you can even get it on the horse in the first place.
I guess armies on both sides of WW1's Western Front trench line had huge amounts of that good f***ing luck then because horses and other animals were routinely masked.
If you'd bothered image googling the phrase "WW1 horses", the second image would have shown you a masked horse. There's even a masked dog on the first page.