Can the word either be subject in a sentence like...Either dog or cat scared mouse.?

No, "either" cannot be the subject in a sentence like "Either dog or cat scared the mouse."

Here's why:

* "Either" is a conjunction. It's used to connect two or more alternatives, but it doesn't have its own meaning or act as a noun.

* The subject must be a noun or pronoun. In this sentence, the subjects are "dog" and "cat," each representing a potential agent of the action.

Corrected Sentence:

* Either the dog or the cat scared the mouse.

In this corrected sentence, "the dog" and "the cat" are the subjects, each connected by the conjunction "or."