I'd be extremely careful in trying to give Ivermectin for horses to your dog, if you get it wrong you could kill your dog. I have dogs and horses and the package of horse wormer warns about only using in horses not dogs. A .21 oz. syringe is enough to treat a 1200# horse! I have a Quarter horse and a 300# miniature horse, it can be difficult to get the dose right for the mini since it is such a small amount. I even go so far as to make sure my dogs can't get into and eat the horse manure for several weeks after the horses are wormed just to sure. That goes for the chickens too who love to work and compost the horse manure.
We get Iverhart Max from
www.kvpet.com for our 5 large dogs that range in size from a Border Collie to a 100# Great Pyrenees. It's basically the generic version of HeartGard and about 1/2 the cost of getting it from the vet.