Children love animals! Well, most of them do, anyway, and if you have an animal in your story, that can help to engage your readers.
If your animals can help to solve a crime, find a missing person or be an asset in some way, so much the better. Equally, if there is an animal in need – hurt, in danger, or otherwise at risk, this could also add emotion to your story.
Dolphins, horses and ponies, cats, dogs, elephants, parrots or monkeys . . . it doesn’t matter really, as long as they help you to bring in lots of action and emotion!