Explaining critters that can be sheared

There is no actual explanation of "how" a critter's grows and is possible to be sheared. Other than drecko and delecta vole, critters don't have anything indicating they need to be fed in order to  shear them, Neither rhex of current dlc, fox nor bammoth of previous dlc explain much.

Current description of critters:
  

At least drecko described you need to have one in hydrogen in order to grow fiber