Or, very large Gorilla.
Most of the positive and negative things about this episode were brought out by the other commentators. I have to add my praise for Janos Prohaska: who expertly and believably performed the gorilla, he also performed the Horta in the Star Trek episode "Devil in the dark", the Mogatu in "A private little war", a beaked bird-creature in "The Cage/Menagerie", and the rock-monster Yarnek in "the savage curtain" where Kirk meets Lincoln. Janos was the indispensable performer during 60s television whenever something unusual was required, but usually he did gorillas, and he did gorillas well. Janos would have loved working with Peter Jackson in the motion capture suit, and the most modern salute to Janos' type of acting has been done by Andy Serkis, who played Gollum and King Kong... although Andy did not put on a gorilla suit, his motions and inflections drove the animations for those characters. Sometimes a special effects person was born in the wrong era: Ray Harryhausen, Willis O'Brien, imagine what they could have done with modern techniques. The fact that they did these incredible things in the 30s and 40s, was inspiration to all modern filmmakers. And so Janos was brilliant in every show where he donned a strange costume. You can even see him in the final episode of the outer limits, as an overgrown microbe-pustule, which looks very similar to the Horta. So it is very true that Janos totally stole this PM episode.
When Della and Perry are driving past Benjamin Addicks' house, there is one very alarming moment when Della screams and the camera focuses upon a lit window, and it zooms in. It is not the appearance of the gorilla that is frightening, it is the way the camera zooms in towards the window. The zoom shot cuts to a the window where the gorilla is running amok. This was cleverly done and the first time I saw this episode, it really did make me jump. It was one of those, not Alien chest burster moments, but earlier in Alien when John Hurt is looking directly into the alien egg and the finger-monster (facehugger) jumps out and attaches itself to his face, it is the same kind of startling thing as this gorilla-in-the-window reveal.
This is probably one of the funniest Perry Mason episodes ever made, and very unusual. Victor Buono plays another one of his myriad characters. This time he is able to stand up vocally against Perry, and you have to admit that Perry's irritation was brought about by Victor's needling, as Perry was not known to raise his voice. Although he did on occasion. Although it was not normal for Perry to get agitated or raise his voice, after dealing with a character like the one played by Victor Buono, Perry's response was justified. Perry always would rise to whatever occasion he had to deal with, Perry even wrestled a gun out of an antagonist's hand during the first season while Tragg took pot shots (The case of the moth eaten mink- and it frightens me that I have the names of these episodes memorized).
I don't know if it was deliberate that there were so many references to previous Raymond Burr gorilla-films, and I also don't know what it means that Raymond was in so many movies that had something to do with gorillas... nevertheless, this episode was a fine and amusing Perry Mason diversion