What is Лошадь in the accusative case?
I looked this up and according to the rules, it should be лошадя.
But I looked up the actual word, and it was Лошадь. Why? Why does it not follow the rules? Is this not the rule for a masculine noun ending in ь?
It is funny, we learned in an intense course, that there is a group of russian nouns that end with ь that are exclusively feminine with one exception. When I was researching for your answer I found this not to be true. But I found one list that might be helpful to you:
So there are two groups of nouns ending with ь. These include all the important ones so you can just learn them by heart or memorize them together with adjectives, this always helps me to get a feel for the language. E.g Какая красивая лошадь!
Gender is fairly easy and strait-forward in Russian. Nouns ending in consonants are masculine. Nouns ending in -o or -e are neuter. Nouns ending in -a or -я are feminine Студент is masculine. Студентка is feminine. Пиво (beer) and море (sea) are neuter. Some nouns ending in -а / -я obviously have to be masculine (папа, дядя = papa and uncle). 10 words ending in -я are neuter, but you only need to pay attention to two of them имя (name) время (time). Words ending in -ь require a little bit of work. Some are masculine словарь, учитель (dictionary, teacher) лошадь, площадь, тетрадь, -ость/ность частность (horse, square, notebook, privacy). For this set you will just have to remember, but you will develop a feel for these words as you grow with Russian. It would be wrong to say that one set or the other is an exception to the rule. Then there's the word кофе which used to be masculine, but is now also recognized as a neuter word. Both чёрный кофе and чёрное кофе are right. So the rules for лошадь are: лошадь in the nominative and accusative, лошадью in the instrumental and лошади in the other three. Some grammars call this the second feminine.
In almost any language that has cases there are going to be words that do not follow standard grammatical rules, usually either due to atypical etymologies or exceedingly common folk usage preceding formalization of grammatical usage in the first case. Лошадь is one such case. If you insist on regularity, you can use конь instead, though it has a slightly more masculine implication.