A guest user profile is designed for public users who access your community. To allow guest users to view or submit data to a standard or custom object, modify the object’s permission in the community’s guest user profile. Each community has a separate Guest User license, so you can control access to objects (including lookup fields) on a per-community basis.
It's not listed on general profile page. You need to follow this approach to reach at related guest user license: https://help.salesforce.com/articleView?id=000212470&type=1
Managing the Site Guest User Profile works just like managing a standard profile with a few exceptions:
You can only assign the permission to read/create on standard objects, but can assign full create-read-updated-delete (CRUD) on custom objects.
By default, the Site Guest User does not have access to any objects, fields, or VisualForce pages. You need to enable permissions and page visibility that are applicable to your use case.
Once you have set permissions for the object, check out field level security to make sure that the appropriate fields are available.
Now, it is important to remember that Site Guest Users can’t be used for any purposes other than Force.com Sites. You can create up to 25 sites (Enterprise edition), which means you can create up to 25 Guest Users. These users to do not count against your purchased user limits, but you do need to monitor the profiles for all of them. If you add a custom field on the contact record that your volunteer application needs to write data to, make sure your Site Guest User has access to that field!
