From Zappa Wiki Jawaka
Elliot Ingber was an American rock guitarist.
He joined The Mothers early in 1966, shortly before they signed with Verve. Ingber's guitar can be heard on the albums Freak Out!, Lumpy Gravy, You Can't Do That On Stage Anymore Vol. 5, The Lost Episodes and Mystery Disc.
He also recorded on the Boy Wonder I Love You sessions.
From the "biographical trivia" section on the Freak Out! (1966) album:
... Elliot digs the blues. He has a big dimple in his chin. We made him grow beard to cover it up. He just got out of the Army. Lucky for the Army.