This particular delicacy isn't entirely limited to Ottawa, although it is rare elsewhere. I have yet to hear about a beaver tails in the USA. In all my travels through Canada, I have seen one stall at the top of a ski slope in Blue Mountain, another hiding at the top of a massive outlook on old Quebec city, and a few in Ottawa (I forget exactly where) and at least one in Toronto.
As for the delicacy itself, it must be tasted to understand. There are those in the world who have lived, and then there are those who have never tasted the sweet triumph that is Beaver tails. Excuse me for going nuts about it but its really, really good food. What is it exactly? A flat pastry in the shape of a beaver tail, dressed with all sorts of confectioners sugar, brown sugar, and secret hobo spices. Then they deep fry it for that artery clogging goodness you wait all your life for. The grill creates a criss cross pattern so it resembles the tail of a beaver. Kudos to whoever invented it.