What is "mad hatters"

Usage examples of "mad hatters".

And he touched his leatherette ticket-folder and dreamed of topiary clipped into the shape of March Hares and Mad Hatters, of frogs in footman's livery and strident queens.

And through these rooms the guests ran, drunk at last, among the robot fantasies, amid the Dormice and Mad Hatters, the Trolls and Giants, the Black Cats and White Queens, and under their dancing feet the floor gave off the massive pumping beat of a hidden and telltale heart.