There have been many who have claimed to be the Mahdi. But there is a sect in India called as Mahdavis who claim that the Mahdi came before approximately 500 years & his name was Muhammad ibn Abdillaah Jaunpuri (referring to the city of Jaunpur in India). I was trying to search for Fataawa about this person but i didn't come across any in 'Arabic or English, though i found enough material about others who claimed to be Mahdis.
I request from the brothers who have information about this person & his sect to post any fataawa related to them since this sect has become very active in India especially with the dawn of the internet & many brothers from India keep mailing me asking about the ruling of the scholars on this sect, askign whether they are Muslims & can one pray behind them or reply to their salaams.