Your guess is as good as mine. Maybe the gun is to protect the patient from the ducks? I have no ideas when it comes to the tiny bowling pins though.
If they were life sized bowling pins I would guess anesthetic? Hit you over the head with one to knock you out & if that didn't work on the first try use the second one.....
Could be. Things like that work pretty well for looney tunes. Maybe. I guess all the tools can be used to check the horse out before you decide it needs to be put down.