Translation:The beautiful horses have small tails.
The sentence "eo: La belaj ĉevaloj havas malgrandajn vostojn." means that every of the horses has (many / couple of) small tails. As we live in the world where every horse has only one tail, it should be "eo: La belaj ĉevaloj havas malgrandan voston."
See more about this at http://bertilow.com/pmeg/gramatiko/unu-nombro_multe-nombro/uzado/specialaj_okazoj.html
Because ĉevaloj is plural. If an adjective describes a plural noun, then the adjective takes the plural suffix as well.
I'm glad this sentence exists, here's why. Earlier there was sentence something like "The children have horses", and comments were that those are rich children. I have explained that the sentence can mean that every child (from that group) has more horses, but it can also mean that every child has one horse, or even that there are more children than horses, so it doesn't have to be that the children are rich.
In this sentence "The beautiful horses have small tails", it can mean that every horse has more tails. But, let's be real, one horse (usually) has one tail, two horses have two tails, so more horses have more tails.