* "Seems" is a linking verb. It connects the subject ("dog") to a description of the subject ("fierce"). Linking verbs don't show action, they describe a state of being or condition.
* "Fierce" is an adjective describing the dog's appearance.
To make this sentence have an action verb, you could rephrase it:
* Her dog growls fiercely. ("Growls" is an action verb)
* Her dog barks at strangers. ("Barks" is an action verb)