Bunny girls usually wear a costume (bunny suit) in which the fabric covers the area from the crotch to the breasts, exposing the shoulders, arms, and legs.
Reverse bunnies are the exact opposite: their shoulders, arms, and legs are covered, while their crotch to chest area is exposed.
There is of course a male version, although it has fewer submissions than the female version.
The reverse bunny craze began in early 2020 and became an unprecedented boom in March.
Incidentally, similar ideas, such as reverse bikini armor and reverse swimsuits, have existed for quite some time, and the oldest reverse bunny itself was posted on pixiv in 2018.