double tailed dog

Would you like to buy a dog with a tail at either end? 
He is quite the strangest dog there is in town. 
Though he's not too good at knowing 
just exactly where he's going, 
He is very very good at sitting down. 
He doesn't have a place to put a collar, 
And I'll admit it's rather hard to lead him, 
And he cannot hear you call 
For he has no ears at all, 
But it doesn't cost a single cent to feed him. 
He cannot bite, he'll never bark or growl, 
Just scratch him on his tails, he'll find it pleasing. 
But you'll have to take him out 
For twice as many walks, 
And I'll bet that you can quickly guess the reason.
-  Shel Silverstein