Thelonious seems to be not as smart as Farquaad's other henchmen, he cannot count and needs help with simple problems. Despite his loyalty to Farquaad, Thelonious is overall pleasant when his boss is not around such as; clapping at Shrek and Fiona's reunion, singing with Gingerbread man at the party. He is also shown to be remorseful when he apologizes to Fiona while he was kidnapping her under Farquaad's order.
Overall, his behavior suggests that he works for Farquaad more out of loyalty and fear then malice.
In the beginning, he was torturing Gingy (also known as the Gingerbread Man). When Lord Farquaad tried to decide which princess to find and marry, Thelonious told him to pick #3 (even though his fingers say 2). During the wedding, Thelonious held cue cards for the audience to react to. When Farquaad was eaten and Shrek and Fiona confessed their feelings, Thelonious was touched, raising a cue card saying "aww." He later appeared at Shrek's karaoke dance party.
Thelonious also made an appearance in Shrek 4-D. In this movie, the spirit of Lord Farquaad enlisted Thelonious kidnap Princess Fiona. At the waterfall scene, Fiona fights Thelonious off her raft and Farquaad abandons him to his fate. When Shrek and his friends go over the waterfall, they survive by grabbing Thelonious, who was in turn grabbing from a branch
Donkey then summons Dragon, who rescues them all including Thelonious and burns Farquaad, thus banishing the evil spirit back to the underworld.
Thelonious is never seen again afterwards, but it is safe to assume without Farquaad's ghost he simply parted amiably with the group.
Thelonis is the penultimate boss of the GBA game, Shrek: Hassle in the Castle, battling Shrek at Farquaad's wedding. After Thelonious's defeat, Farquaad battles Shrek but notably he is significantly weaker then his henchman, having less then half the health of Thelonious.
Thelonious appears in Shrek Super Party as a playable character.
Thelonious appears in Shrek: Fairy Tale Freakdown as an opponent and unlockable champion. His attacks include throwing hand axes, charging at his opponent and punching.
He also appears in the Nintendo DS version of Shrek Super Slam as a playable character.
The name "Thelonious" is a parody of the word "felonious".