"The horses are not the best."
Translation:Los caballos no son los mejores.
Why is it "son" and not "estan"? I feel like being the best is a relatively temporary status, not a permanent attribute.
The difference between ser and estar is not permanent vs temporary. Please forget you were ever told that. It's intrinsic nature or identity (ser) vs state or condition (estar). Identity may not be permanent, and condition may not be temporary!
This sentence says that two groups of horses (the horses and the best horses) are the same. Grammatically, that's a statement of identity, no matter what other information you gain about the horses. So it's "son," not "están" for that reason.
I do consider temporary vs. Permanent state somewhat. I find using the acronym better PLACE (Place, Location,Action,Condition,Emotion) works for me.
SER=doctor, description, occupation, time, characteristics, time, origin, relationships ESTAR=place, position, location, action, condition, emotion
Being the best is not temporary, at least not in Spanish, 'languagely' speaking.
Why can I not say "caballas" for female horses, and make the "los" into "las" and "mejores" in "mejoras"?
A "caballa" is a mackeral. Much as we have a separate word in English for a female horse ("mare"), so does Spanish ("yegua").
After all, you can lead them to water but you can't make them drink.
I can't understand why "los" is correct here. Why not 'el'? Is it to show that there are a lot of "best" because there are lots of "horses"?
Because it it 'the horseS'. The article must agree with the noun it's referring to. It may be hard for people with English as the first language because articles and adjectives do not have a plural in English. It would be like 'Thes horses are thes bests'. 'El caballos 'is really wrong.
"Mejores" means "best." When used as a noun ("los mejores") it means "the best ones."
"Vencerán" is a verb. It means "(they) win/defeat." Maybe you were thinking of "los vencedores?" That would be "the victors."