There are more than just one concept of Imam Mehdi due to sectarian division in Islam. The Shi’ite(Iran) concept of Imam Mehdi is more like a fictitious story rather than being based on traditions of the Prophet Mohammad. Shi’ite Islam has segregated from the mainstream religion and remains focused on the household of the Prophet Mohammad. Anyone outside the household of the Prophet is given considerably less or no importance at all. They believe the lost son of Imam Askari is the awaited Imam Mehdi.
Sunni Islam focuses on the traditions of the Prophet Mohammad and their concept of Imam Mehdi is strictly based on traditions of the Prophet only.
Wahhabis don’t accept the existence of the concept of Imam Mehdi. They, however, believe that a caliph will come to help them towards the end times.
None of the above mentioned concepts are realistic and authentic.