This is a little bit hard to explain concept, practically some like a kind of symbiosis in which two organisms evolve together one get transport, almost free nutrients and relative security and the other gets the possibility to suplement lost limbs by the body of an specific part of the body from the "living prosthetic".
I think the prosthetic creature specie will get the most benefit as you would be assured of food, transportation, and safety most of the time unless the "holder" has lost a limb and the rest of the time it just rests somewhere inside the holder or wrapped around the body.
But this would require that prosthetic creature would be able to seal a possible wound, adapt to the shape of the limb that the "holder" require and to be correctly used to suplement the lost limb, I meant the moves can be controlled by the "holder".
But I don't about the viability of this and how the different implied mechanisms would work.
The mentioned features are the basics of the concept for this question, the "prosthetic" and the "holder" should be a symbiosis which is almost like be a single creature. The prosthetic not necesarily should have the exactly shape of the previous limb, the "holder" could have lost a an arm but the "prosthetic" turns to a tentacle and in results useful for the "holder" can be kept.
Optional extra point: What kind of scenario would make this viable if is not?