What does "healthy as a horse" mean?

the second answers

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That is not a saying.

As fit as a butchers dog would be more apt.

It means that back in the late 1800s, horses were work animals.  If they could work, they were considered "healthy."  If they couldn't work, they were slaughtered.

Therefore, the uninformed considered horses to always be very healthy, because they couldn't see the lamenesses and thinness and so on that showed they really weren't healthy, and they never saw horses that were bad enough that they could have seen were unhealthy.

As Christine said, the saying is ironic, though the sayer usually doesn't realize it.  Horses can be really fragile.

This statement seems a bit ironic.  I've worked enough around horses to know they are generally NOT healthy.  They're always needing some veterinary or farrier care.  Costly animals.  

Before mechanized equipment like motor vehicles and farm machines, horses were commonplace and used for everything from transportation to plowing to harvesting timber, etc.    Horses were a symbol of strength and health.

Never heard of  it. Strong as a horse yes. Also, hung like a horse. Hope this helps.