The Ape Man in Early Film Comedy

Several early film comedies dealt with a man transforming into a half man and half simian. 

In the 1906 Gaumont comedy The Truth Behind the Ape-Man, a man drinks a new tonic to grow hair on his head but ends up with his entire body covered with a thick coat of hair. 

A 1912 Pathé comedy, Plus de chauves (No more bald men), also involves a hair tonic that turns a user into a shaggy beast. You can find the complete film at https://vimeo.com/410122308. 

The premise was more involved in the 1909 Pathé Frères comedy The Man Monkey. A mentally ill man (André Deed) is arrested by police for his wild behavior and is quickly committed to a sanitarium. A surgeon seeks to cure the man by replacing his diseased brain with a monkey brain. The surgery does not have the intended outcome, as described by Moving Picture World:

Immediately there is a great transformation, and the patient turns into a monkey and begins cutting up all the antics for which this animal is noted. The doctor takes his charge on a leash and starts out, and as they are going down the street the mischievous fellow plays all sorts of tricks, upsetting stands in the market place and causing no end of excitement and amusement to the general public.

The 1911 Éclair comedy Dr. Charlie Is a Great Surgeon copies the Man Monkey plot except the brain transplant is replaced by a stomach transplant. Presumably, our behavior is controlled by the food we crave. 

Cricks & Martin didn't think that something as complicated as a surgical transplant was needed to make a man act like a monkey.  The studio's 1911 comedy The Man Monkey has a man behaving more simian-like simply because he ate a bag of monkey nuts (old slang for peanuts).

Ape-Man films got more serious in the 1920s. 

Go Get It (1920)

A Blind Bargain (1922)

The Monkey Talks (1927) 

During the 1920s, Patrick Walshe became known as "The Monkey Man" while touring the vaudeville circuit with a monkey impersonation act.  He later played Nikko, The Wicked Witch's flying monkey minion, in The Wizard of Oz (1939).