Who is Mammon whom Jesus warned of in Sermon of the Mount? In the beautiful sermon of the mount Jesus said (Matthew 6)" None is able to serve two lords, for either he will hate the one and love the other, or he will hold to the one, and despise the other; ye are not able to serve God and Mammon". 

Who is really Mammon?
From Wikipedia


  Scholars do not agree about its etymology, but it is theorized that Mammon derives from Late Latin mammon, from Greek "μαμμωνάς", Syriac mámóna ("riches"), Aramaic mamon ("riches, money"), a loanword from Mishnaic Hebrew 'ממון (mmôn) meaning money, wealth, or possessions;[8] although it may also have meant "that in which one trusts".


And Gregory of Nyssa calls Mammon the evil one:


  My Brethren, what is the force of these words? It seems to me that the Lord names the evil one in many varying and differing ways according to his evil operations [lit. energies]. Thus, He calls him the "devil" (diabolos-false accuser), "Beelzebub," Mammon," "prince of this world," "man killer," "evil one," "father of lies," and other similar names. Perhaps, then, another name for him is also "temptation" (Gr. peirasmos).