According to this article from Pottermore, Wizards Unite will feature a form of creatures usually found in the Wizarding World of Harry Potter:
The game encourages you to step outside with your phone, explore your local surroundings and take part in a series of adventures, such as searching for magical creatures and bumping into iconic wizarding world characters along the way. Crikey. Who knows who you’ll spot in your local park? You’ll also be able to cast spells, discover mysterious artefacts, and build up your very own, illustrious wizarding career. Yep, we can definitely get on board with this.