My step son, who is a veterinary surgeon says "you could use it for any large animal to hold it's jaws apart for dentistry or whilst administering medicine", i suppose he counts as somebody else!
I now remember throwing one on the scrap here many years ago whilst clearing out old stables.
In use you hold the handle end which hangs below the animals lower jaw and it's nose is free to breathe above the opposite open end, but i've never seen one like this used. The last time a horse here had it's teeth done the mobile horse dentist used a natty modern ratcheting type gag so she had to 'open wide' very nicely! Afterwards the horse was much improved and could eat comfortably without dropping food, and i had a beer with the dentist.
Tools like this are not needed to dose cattle and sheep with liquid medicine as they have a gap between teeth you can fit a bottle neck between.