This page lists all the equipment used by the various companions in Fallout 3. None of these items can be obtained regularly, only by using console commands. They do not have unique names and appear as their regular variants in the inventory, however they have different stats.
The weapons do not degrade and the firearms use their own ammunition types which cannot be obtained by the player character, likely part of the mechanism that gives them unlimited ammunition when in the respective companions' hands. They are interchangeable between the different companions.