A Wheel Dog

A Wheel Dog explained

I was sitting in my chair at the dinner table when Martin came up to me with a creation he had made with Lego.  It was two wheel sets stuck together so they formed a sort of cross.
'What's that, Martin?" I asked.
"A Wheel Dog." he answered proudly.
"A real dog?" I asked, as he sometimes doesn't get his letters correct.
"No," he answered firmly, "A wheel dog."
Then he pointed to one end and then the other.
"See, his wheel head and his wheel tail."
"Ah, of course.  It is a wheel dog."
"Yep!" and off he went very pleased with himself.