Muskrat, Beaver or Nessie?

Someone poked his head through the ice this afternoon - the pond muskrat/beaver (not sure which).   He's pretty small - smaller than the previous pond resident.  I informed Mom and Dad that they can live up to 25 years.  In my estimation he's probably a teen ager - or maybe 20 years old.  If he was 2 years old like dad suggested he would never be able to survive on his own because he's just a baby.  Duh.