「見る・見ます」は現在形で「見た・見ました」は過去形です。英語だと "He saw a horse." になります。英語では過去・現在・未来の区別が日本語以上に厳密なので注意が必要です。
In a normal sentence, the object is denoted with the particle "を". Therefore it can be
This is a valid translation. However there can be another version, as is given by Duolingo,
To explain this, let's start with 「一頭の馬が見えます」. Here the topic (or "subject", kind of) is 一頭の馬 = "a horse", followed by the particle が (which denotes the topic of the sentence). The predicate is 見えます = "is seen". Therefore 「一頭の馬が見えます」 means "a horse is seen". Note that 見ます is "to see" and 「一頭の馬が見ます」 would mean "a horse sees".
Now the English sentence "a horse is seen" is passive, and it lacks the agent. Meanwhile the Japanese 「一頭の馬が見えます」 is not a "passive tense" strictly, but due to the characteristic of the verb 見える it is something you can call a "quasi-passive" that lacks the agent. In such cases you can add the agent, simply by placing 「Agent には」 in the front. Here に is the particle that shows the starting point or agent of some action, and は is the particle for the topic (Either に or は could be omitted, depending on the context, but I won't go deep into it). Hence the sentence.