the cavum vergae followed
by the cavum septum pellucidum.
Cavum septum pellucidum is a normal variant. Because it is filled with CSF, CSP is also sometimes referred to as the fifth ventricle; however, the most common type of CSP is not connected to the ventricular system, and the use of this term to refer to CSP should be discouraged. "
So, if I am reading the medspeak correctly, it can be a normal variation - and if you read up further, it may have been there since birth.