If in a course example Dog extends Animal, it can be an arbitrary demonstration of language mechanisms (with an uncontroversial is-a relationship) but even in that case it is implicitly suggested that it is a good or "normal" design, implying an alarmingly complex program that has good reasons to deal with those two types.
Such a program is usually not described for brevity, giving the false impression that it exists: if the problem were analyzed with any diligence, usually Dog would appear a completely pointless complication.