Nightmares in children are in principle normal, but if they are frequent or very distressing, there may be the need for psychotherapeutic support and for evaluating possible tensions in the family environment. For instance, children start to dream of animals at around 4 or 5 years old and at this stage they assume that the nightmare is very real, without distinction between dream and reality.
Dreaming of animals is natural. If we look at human evolution since pre-history to present days, the animals were potential predators, so dreaming of them was a way to organize learning and the essential defences for survival.
It is also important to distinguish nightmares from night terrors: while there is recollection of the dream in the first case, in the second there is no memory of anything.