змея - женский род - одна "На самом деле мне нравятся змеи, я хотел (завести) одну в прошлом." одну - винительный падеж

"Как сказать по-русски?"

Thanks Liliya, that is exactly why I asked that question. I wasn't entirely sure if I had ever heard the word "one" used in this context. It sounded weird)).

Here "one" is an English pronoun, that doesn't have Russian equivalents. When we translate it we either omit it or say the noun itself, or sometimes we can translate it like "такой" (using appropriate declination). And its not a numeral here. So I'm sorry, but I can't agree with the previous answers or comments (except Selyd's, his translation is very nice). I would say: "Вообще-то, мне нравятся змеи, в прошлом я хотел завести змею."

"Мне, вообще-то, змеи нравятся и я хотел было такую."