A: If you were in BC you could possibly move to end this tenancy under section 47 identifying that the tenant is unreasonably disturbing other occupants of the premises. But your problem is that you agreed the tenant could have the dog, now you've changed your mind. Here, the onus would be on you to persuade an Arbitrator that this tenancy must end and the tenant should take her problem to someone else's property. Tough sell, if you ask me and I think you are.
As long as the tenant is paying the rent on time and not contravening any legislative provision in the Ontario Act I think you're likely hooped. You're going to have to wait this one out, cut a deal with the tenant, offer her some incentive to move on, or figure out why the dog is barking all day long and address that problem. It may well be the best investment you can make to bring in a dog handler/expert/obedience type to determine why the dog is barking and how you can get it to stop. It ain't fun I can bet. But, as the poem says: you've made your own bed and now you've got to lie in it.