Fluffy

 are two Radroaches caged in the Sniper Shack in 2277.

Other interactions

 * If released, they will be hostile to the player. After they are released the player can get them into their cage again by standing on the cage with the door open so they will follow the player inside their cage.
 * Despite being named creatures, they will respawn if they are killed.
 * If the player gets too close to the cage, the radroaches can attack through the cage bars and deal damage.

Appearances
Fluffy and Jitters appear only in Fallout 3.

Bugs?

 * of course they are

Behind the scenes

 * Fluffy and Jitters have different names in some localized versions of the game.