WhatsApp Beta for Android has finally added support for fingerprint unlocking on its latest update. The update also bumped the app version to v2.19.221
The support for fingerprint unlocking on WhatsApp for iOS is present for a while now. But you can finally expect it to Android pretty soon. For beta users, if you want to enable Fingerprint authentication, head over to Settings > Account > Privacy to enable Fingerprint Unlock.
Make sure that your Android phone is running on Android Marshmallow or above. WhatsApp will also hide your messages when the app is locked so you need not worry about your privacy.