The complete subject in the sentence "My neighbor’s dog barked all night" is "My neighbor’s dog."
In the context of a sentence, the subject is the main entity that the sentence is about—the doer of the action or the one that the action is happening to.
It typically consists of a noun or pronoun along with its modifiers. In the sentence "My neighbor’s dog barked all night," "My neighbor’s dog" is the complete subject, as it includes both the possessive noun "My neighbor’s" and the noun "dog."
The subject is what the sentence revolves around and provides the focus for the reader or listener.