Isaac is shown to be territorial and hostile to any other dog who approaches him trying to take his food. However, he is shown to have a fear of dogs infected with rabies after Tramp tells Isaac that Lady has rabies.
Role in the film
When Lady escapes the pet shop from Aunt Sarah, she survives the traffic around the road to which Isaac came face to face around his dog food, he was extremely territorial when they wanted the food to win. Tramp suddenly interferes with him, where he needs to stop when Lady frightens him. When Lady pretends to be a rabid dog and Tramp tells him she is infected by rabies, Isaac runs off in panic, leaving his food behind while Lady tells Tramp to get the muzzle off her mouth. Isaac's further fate is unknown.