Delete your account
Sign in with the account email and password. Your creature, your journal, your goals and your history all go. There is no way back.
If you want a copy of your data, do it in the app first — once erased, it is gone.
Deleting here does not reach your phone or the Android backup, which stays in your Google account and comes back if you install the app again. Both go with "Erase everything", under Your data, inside the app: what is on the phone right away, and the copy at Android's next backup, with the app still installed. Without the app, the copy goes from your phone settings, in the backup section: turning off the phone's backup — which applies to all apps — deletes the backups kept in your account, according to Google.