Dean Pelton's various outfits are a running gag on the show that first appeared in Season One and became a recurring joke starting in the Season Two episode "Accounting for Lawyers". The outfits are usually worn when he announces something to the study group, although there are few exceptions. It's revealed in the Season Three episode "Course Listing Unavailable" that Pelton has a huge closet in his office containing many outfits for him to choose from. Below is a gallery of all various outfits he has appeared in with descriptions along with the seasons and episodes that they made their debut.
Dean Pelton wore this outfit on Halloween during the study group's first year at Greendale. It was modest in comparison to the outfits he eventually wears, just a tuxedo with a masquerade ball mask.
A cartoon is shown in the episode end tag which has Dean Pelton transforming into "Mega Dean" and killing the study group. It seems to have been created by the Dean himself who voices all the characters in it.
The Dean outdoes himself dressed as "Duali-Dean", referencing the duality of man and the dual news both good and bad about their upcoming Biology class.
A flashback by the study group shows Dean Pelton dressed as a fireman in order to warn the study group of a fire that had just broke out in the building.
A flashback reveals that Dean Pelton dressed up like a janitor in order to deliver the study group their share of pizza on the cafeteria's "Pizza day".
Dean Pelton wore this outfit when he went to bail an amnesia suffering Ben Chang from jail. Believing his amnesia to be genuine, Pelton said he wanted to nurse him back to mental health.
Dean Pelton wore an outfit similar to Jeff's cowboy costume and greeted the study group with a "How-Dean" before telling them about the "Thanks-Living" potluck dinner the school was throwing.