Joe Beck is name-checked on the cover of "Freak Out!" (1966) under the heading "These People Have Contributed Materially In Many Ways To Make Our Music What It Is. Please Do Not Hold It Against Them".
First I thought of Joe Beck "the actor" who featured in or contributed (his voice) to "Bonanza", "Hawaii Five-O", "The Twilight Zone", "Spider-Man",... but that was beyond 1966, when the "freak list" was published. Then I thought of Joe Beck "the producer", known for the English version of "Kingu Kongu Tai Gojira" (1962), a.k.a. "King Kong vs. Godzilla", originally made by producer Tomoyuki Tanaka, with director Ishiro Honda. Beck worked with Thomas Montgomery (director) and Paul Mason (screenplay) on this one. In 1966 he produced a movie called "Cyborg 2087"... Beck also produced "Journey to the Center of the Earth" (1959) with James Mason, Pat Boone, Arlene Dahl, and Diane Baker (also name-checked on the cover of "Freak Out!") - based on a novel by Jules Verne (directed by Henry Levin).
But then again, since also Bob Reiner is name-checked on the cover of "Freak Out!", it had t be one of the members of The Leaves. The Leaves - Bobby Arlin, John Beck, Jim Pons (!), Tom Ray, Robert Lee Reiner, and Bill Rinehart - were one of the first LA folk-rock groups to spring up in the wake of the Byrds in the mid-'60s. The Leaves are most remembered for recording the first - and one of the most successful - rock versions of "Hey Joe", which reached the Top 40 (and was a huge Californian hit) in 1966. None of their other releases approached this success (although "Too Many People" was a local hit), but the group recorded a fair number of strong covers and original songs during their brief existence. More explicitly Rolling Stones and Beatles-influenced than the Byrds, they didn't project as strong an identity as competitors like the Byrds or Love, despite displaying considerable talent for harmony rockers in both the folk-rock and British Invasion styles. After cutting some singles and a decent album for the tiny Mira label, they moved to Capitol, and disbanded after a disappointing follow-up ("All the Good That's Happening", 1967) that offered less distinguished material and a more diluted sound. Leaves bassist Jim Pons went on to join the Turtles for a while in the late '60s.