I don't know about them being a myth or old wives' tale, but I have several that I use for wounds that work as good and sometime better than products purchased at a feed store or special horse store. Most folk actually have these on hand!
On leg wounds and body wounds - when they first happen and have been washed - you can pack it with plain old table sugar. The serum from the wound reacts w/ the sugar and will actually repel flies while healing. Hair seems to grow back faster, too, once the wound is closed.
on lower leg wounds - to help faster healing and prevent proud flesh - we would scrub the wound with a brush, scrubby or even a new toilet brush (the vet clinic I worked at kept them on hand). Then they would be packed inside w/ meat tenderizer (be prepared - it HURTS - and some will raise leg sharply or they will kick out hard) and then put Preperation H around the outside perimeter of the wound (promotes "shrinkage" and helps the edges come together much faster - also preventing proud flesh from forming).
I was told, as a kid, to use plain bacon grease on wounds or scrapes to promote hair growth. Our family kept bacon grease separate from other fats/greases and we cooked bacon every weekend so we always had a supply of "clean" bacon grease. It was messy but it seemed to work. Now, with my family, I don't have a tendency to do as much bacon and we don't keep it separate either so we don't use it.
The over the counter horse coat product MTG smells somewhat like bacon grease/barbque but i understand the scent is from sulfur not grease. It is greasy like bacon grease though.
During the winter, in CO, when riding we usually packed our horses' hooves with lard. We kept a tub in our outdoor fridge (kept temps more even then out on the shelf in the freezing weather in the tack room) and used it before taking horses out in the snow once saddled. If we noticed hooves "balling up" on pasture horses, we'd pull them in and pack their hooves as well. Funny - haven't used it here where lard is much more plentiful - but then it doesn't so much "ball up" as just freezes solid here...
Either olive oil or even WD-40 sprayed/combed thru the tail/mane to remove cockle burrs.
I know of others... cant' think of them right now.