Custom Fields are treated system generated field

There are some fields that the system treats them as system generated field. I am not sure how the previous developer created them. When I go to Doctype and see the default fields, those fields are not present, but they are on Customize Form.

When I try to Customize Form and remove those custom fields, it give me this message

“Cannot delete system generated field Meta Keywords . You can hide it instead.”

I was wondering how were these fields created and how can I remove them?

Thank you!