It's not a problem to talk to a horse...only the horse will not reply to you (in words). But can execute the command. (start moving, stop moving) Also... For talking to horses read the "rangers apprentice" books. And I suppose I 'll find a horse somewhere , talking, in the books from CS Lewis (Narnia)....There this sentence makes sence.
It's in the instrumental case. Masculine singular nouns take the -em or -iem ending, feminine singular nouns take the -ą ending, and all plural nouns take the -ami ending.
The instrumental case is used with the word być (to be) in all its declensions on the noun that the subject is described as being. For example,
"Jestem nauczycielem" - I am a (male) teacher. "Jestem nauczycielką" - I am a (female) teacher. "Jestem polakiem" - I am a polish man. "Jestem polką" - I am a polish woman.
It's also used similarly with the word "zostać" meaning "become". "Chcę zostać lekarzem" - I want to become a doctor.
Predicate Nominative refers back to the subject of the sentence. There is
an equivalent of it in Polish. You find it as a dictionary entry and you use it when you present or indicate clearly obvious thing or matter:
IT is a horse! (IT = horse) - TO koń!/ TO (jest) koń! (TO = koń)
Horse is a horse! - Koń TO koń!/ Koń TO (jest) koń!
(Nominative case/Mianownik (kto? co?) koń
You don't know me... I (the subject of the sentence) have to explain, who I
am by using a different noun, the object (NOT a subject), which has to be
in Instrumental case. That is how koń IS/BECOMES koniem in Polish :)
I AM a horse - (Ja) JESTEM koniem
Horse IS a horse! - Koń JEST koniem
(Instrumental case/Narzędnik - (kim? czym?) koniem
Jellei once told me that the creators adapted this phrase from the Dutch course, but I do not remember under which sentence it was. It likens this sentence, nonetheless. :D As for using the verb steal, I'm sorry, I did not mean to refresh any clichés, I just don't think that much cooperation between the user-driven courses happened, or at least I cannot imagine it, although it sounds feasible, in terms of enhancing one another's course. :D Still, the citizens of Brandenburg, our easternmost federal state, and one of the two states that shares a border with Poland, usually complain about thieves and burglars from Poland who break into houses and run away across the border with their loot. I think that is where such clichés are founded upon. :-(