Does an expiatory offering (kaffarat yamin) become obligatory for me if I swear, saying: "By Allah, I will never eat chicken," and I intended a specific chicken, but then I ate a different chicken other than the one intended in my oath?

An oath is based on the intention of the one taking it, and his intention specifies and restricts it. So, as long as you intended the chicken present on the dining table at that time, you do not break your oath by eating other than it, and no expiation for the oath is required of you.