In English, countable nouns always need a determiner in the singular. The determiner can be the indefinite article, the number one, the definite article, a possessive adjective, a noun in the possessive case (’s), a demonstrative adjective (this / that), or indefinite determiners such as “some”, “any”, or “none”. Singular countable: This is a pizza. (That is, an entire pizza pie.) In the plural, or with uncountable nouns, you can’t use the indefinite article, but you may use the other determiners, including indefinite ones such as “some”, “any”, or “none”. Plural: These are (some) slices of pizza. Uncountable: This is (some) pizza.

The answers are: It's a snake and It's a neck. It's grammatically incorrect to remove the article 'a'

Phil gave a good general answer. As an addition, "THE neck" is also correct (if looking at the whole animal). For example, when looking at a horse... What's this (called)? The neck (the only neck). What's that (called)? A leg (one of four legs).

a snake a neck because they are countable singular

Thank you for your answer.