It seems like you have multiple custom object with the same name. By default, there is only 1 Base object. Try to rename the custom ones and you will be able to see the original one or the required replaced base object.
Hi Nicolas, I see you are orking with French language. I've had similar issues working with PT version were the translation is not so good and a several objects get translated with the same name. I've switched to English and then I could find the correct base object.