Question

what walks up the mountain with three legs and comes down with four legs

Answer 1

A dog, with a broken leg....When it goes up it can use only 3 of its legs.So it goes up the hill with 3 legs...The first part is answered. 
The reason why the dog is going up the hill is becoz there the vetinery Doc is living on the hilltop and he runs a clininc there, and he never comes down.He goes from place to place in a Helicopter which he keeps in his shed behind the clinic.So there is no point in waiting for the Doc to come down!!! 

So the Dog gets to the Doc...the Doc heals the Dog..and the Dog is allright again....Now he comes down the hill with the 4 legs