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