It's the dog tote from Cycletote.
www.cycletote.com. It was pretty pricey, as all carts are, but it is a fantastic cart which you can customize with brakes, awnings, etc. Ours has 2 additional sets of "bars". each one replaces the bike attachment: one is to enable me to push* the cart if I don't have a bike and the other is for my Bernese Mountain Dogs to pull the cart like they do in the swiss mountains.
It's made really well and comes in different sizes. Mine was for my 100 lb pyrenees/golden mix. He lost use of his rear and could only walk about 15 feet without falling (at his worst). I rehabbed him, but the problem was that he would walk with eagerness only to lay down and quit. I would have to leave him tied to a tree to get the car, and I wasn't comfortable with that. So with the cycletote, me, him, and the bike would walk as long as he wanted and then I'd throw him in the cart for the ride home.
He was up to a mile a night right up until he died, 3 years later.
However, I see now that Burley makes a dog trailer, and it looks cheaper. I don't know how big it is, though.