The subject of a relative clause can exist in several different forms.
It may be the relative pronoun itself (that resides in the subject position in the relative clause):
Wendell attacked the man (who was feeding donuts to the filthy, overweight pigeons.)
It may be a pronoun (that resides in the subject position of the relative clause):
Eating with his mouth open was a trick (that he had learned in the army.)
It may be a generic noun that resides in the subject position of the relative clause:
Donald knew that the toy collie (that his daughter had moved in with) would never be able to support her.
It may be a gerund phrase that resides in the subject position of the relative clause:
The exercise (that running in the ocean was) proved to be too much for Harold's little gerbil heart.