If someone had acquired the response “dog” in the presence of several different types of dogs and then saw a fox for the first time and said “dog,” that would be an example of metaphorical extension. The response form is controlled by the many features a fox has that are the same as many of the relevant features of a dog.
An instance of verbal behavior with the following features:
The response form has been previously acquired
The stimulus must be novel
The novel stimulus has some, but not all, of the relevant features of the stimulus that previously controlled the response
The type of elementary verbal relationship between the stimulus and the response
Formal and dynamic characteristics of both the stimulus and the response
Whether or not the current response is followed by reinforcement