The Hungarian sentence conveys a general meaning, horses in general are important. It should be accepted both with & without "the", imho.
You may wish to review the section on "Generic statements" in the tips and notes to the "Adjectives" skill: https://www.duolingo.com/skill/hu/Adjectives-1
(And if you're not reading the tips and notes - perhaps because you're learning through the app - you're missing out on quite a bit of helpful explanations.)
From the lesson's notes: "While in English you don't have to use an article for the subject in such sentences, in Hungarian you usually do."
It says "usually" - what are possible exceptions to this rule?