Mahan came from the word “mahthan”, a Rajput (later called Sisodian) title or name, today still many Rajputs have sirnames “Mahan”. See
“… a present-day bearer of the surname “mahthan” belongs to the mahthan shakha of the larger Sisodia Kula, which in turn is a subset of the larger Gehlot or Guhilot clan, in turn part of the Suryavamshi lineage. Furthermore, all mahthan belong to the patrilineage of a certain rao mahthan, who was a direct male-line descendent of a definite male ancestor (in this case, Rana Hamir of Mewar) who belonged to the patrilineage of a certain Guha, believed to be the father of the Guhilot clan.” — Rajput clans
Further readings on the Rajput clans:
James Tod, “Annals and antiquities of Rajasthan, or The central and western Rajput states of India (1920)
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Mahthan is real Mahan