What's so special about horses, eh? Must they insist on riding about with their species name stencilled on their wagon in huge letters? Prima donnas, the lot of them.
And why is it always in uppercase shouty letters like my mother's text messages? Perhaps they haven't figured out how to use the shift key on their signwriters. Or perhaps they're really dangerous - mortal danger tends to warrant big letters, I would think. Or they're all extremely full of themselves. I don't know about you, but I won't be taking the chance of finding out.
Besides, it's a missed chance for a slogan, a bit of self-promotion. I mean, if you're going to go to the trouble of putting "HORSES" on there, you may as well pop on a few extra words and make a statement about it. Like "Try Riding a Cow" perhaps?
I think in the interests of animal parity we should extend the gesture to all forms of animal transport. Imagine the lorry with the bold slogan "Cows - Great milk, no gimmicks" or even "Pigs - The Gift that Keeps on Giving."