Humans have been killing animals for thousands of years, and with the development of the self-contained cartridge, the Greener company started making a compact and efficient Humane Horse Killer. Used by veterinarians for euthanizing creatures (versions were made for pretty much all major domesticated animals), they were made into the 1960s. This type of device is known as a "free-bullet" design because it uses a traditional cartridge, as opposed to the captive-bolt designs which maintain positive control of the lethal end of the device and retract it into the unit after firing.
This particular Greener model is one of the more commonly encountered types, as it was used by the British military (which used horses in great numbers in both World Wars) and was a standard piece of equipment for troops tasked with overseeing care of those horses.
VIDEO HERE (9:25 minutes)