Reference to http://www.sacred-texts.com/bud/bits/bit-int1.htm,
it says :
The Buddha's given name would appear to have been Siddhattha; but
as the word means "Successful in his Objects," it looks as though
it might be a simple epithet. The p. 2 Buddha belonged to the Sakya
clan. The word "Sakya" means "Powerful;" and the families that bore
the name had a reputation for pride and haughtiness. They were of
the warrior caste, but cultivated the peaceful arts of agriculture.
By his contemporaries The Buddha is usually called Gotama, or, as
the word is sometimes Anglicized, the Gotamid. It is not quite
clear why he and others of his clan should bear the name of Gotama
in addition to that of Sakya. It may be they claimed descent from
the ancient sage Gautama (Sanskrit "Gautama" becomes "Gotama" in
Pâli), to whom are attributed some of the hymns of the Rig-Veda; or
it may be, as Burnouf has suggested, "because Gautama was the
sacerdotal family name of the military race of Sakyas, who, being
of the warrior caste, had no ancestor or tutelar saint like the
Brahmans, but might, as the Hindu law permits, have taken the name
of the sage to whose family belonged their spiritual guide."
