In Harry Potter Wizards Unite you must free, find or fight Foundables in order to send them back to the Wizarding World.
In the Exploration tab of your Registry, you'll notice that the Oddities category is a little special. Foundables in this category must be knocked out in order to recover them.
There's only one problem — they only appear at certain times of the day.
For Werewolves, the lore of Harry Potter (and fantasy in general) is followed closely, and they only appear when there is a full moon.
To facilitate "capture", it is possible to find Werewolves two days before and two days after the full moon phase (and of course at night only).
2019 Full Moon Phases
Next full moon period: August 13-17