The ability to problem solve is a high level sign of the capacity to
reason. With this said, then dogs do have the ability to reason. The
breed of the dog, according to Stanley Coren, has something to do with
the level of intelligence the canine might acquire.
According to Stanley Coren in his book, The Intelligence of Dogs, the
towel test is the quickest most telling way to view the level of
intelligence of a particular dog. The way the test works is you toss a
towel on the dog's head, and the speed in which the dog removes the
towel shows the level of intelligence. The quicker the "problem" is
solved, the higher the IQ. According ot the book, the border collie
ranked as the highest, while the basset hound was one of the lowest.
(One has to wonder whether the basset hound really cared or not).
There are several tests in the book to determine the IQ, and I found a
site, which I linked to below which adapted these tests to use with
horses, so you can go through them yourself
Equine Social Intelligence Tests (Created for horses from the Canine
tests by Stanley Coren)
The Intelligence of Dogs by Stanley Coren