Since you can't tell, without context which we don't have here, my guess is that both "horse" and "horses" would be accepted.
Why "stará" is both "old" and "take care of"?? Can someone explain the logic behind that?
I am quite curious if in Czech "Starat se o" means "take care of" or "care about", as you know in English they are quite different behaviors....
What is called here the grammatical case of "o koně"? How would you say "he takes care of cats"?
Isn't it accusative here? Wiktionary says koně for accusative for both singular and plural, while koni/koňovi and koních for locative. https://cs.wiktionary.org/wiki/k%C5%AF%C5%88
Same with the cat. Kočku and kočky for accusative, kočce and kočkách for locative. https://cs.wiktionary.org/wiki/ko%C4%8Dka