"Лошадь" is not "dog"!
In some lessons, there are sentences, that contain the word "лошадь", but there is no word "horse" in the word bank. Instead, there is the word "dog", and it is counted as correct. Dog is "собака" in Russian, not "лошадь".
I think you are coming up against idiomatic usage versus literal translation. In English when you want to communicate that you are working hard you can say, "I'm working like a dog." In Russian dogs don't work hard - apparently - but horses do. So if you want to say that you are working hard in Russian you say that you are working like a horse, "Я работаю как лошадь"
This is a really good sentence. In Russia a dog is собака, not лошадь. But if they want to express a certain frustration, they could say about everything or everybody, - Вот, собака! What a dog. For example, meaning the horse, - Вот собака! Она не хочет есть сено. What a dog! It doesn’t want to eat hay.