When it relates to canine breeds, generalizations are impossible to make. However, the temperaments of most dogs of a particular species will be similar. As a result, the most difficult dog breeds to housetrain are Jack Russell Terriers and Yorkshire Terriers. The fact that these 'harder to teach dogs' are also small dog breeds is unsurprising. Potty training little and toy dog breeds are notoriously difficult. Their smaller stature and rapid metabolism are often blamed for this.