Good question, Stephen. While there is no information on reports of toxicity from Aronia melanocarpa or black chokeberry at the current time, other plants that belong to the Rosaceae family can contain cyanogenic glycosides in varying amounts. These glycosides, if ingested in large enough amounts, can produce difficulty in breathing, dilated pupils, brick-red mucous membranes, hyperventilation and shock. Because of this, it is a good idea to discourage your dog from nibbling on this plant.
As for buckhorn, there are a number of different plant genera that are referred to by this common name, each with varying degrees of toxic potential. In order to provide you with the most relevant information, we will need to know the scientific name, that is the genus and species, of the buckhorn you have. If you are not certain of this, you might want to bring a cutting to your local nursery or garden center for identification.