While not as big a part of his life now, the helpful little alien was at one time....
....here are a few of his appearances!
Detective Comics 311
Zook even detects another Saturnian alien disguising himself as John Jones (the Martian Manhunter's Earth identity) in Detective Comics #314, and even briefly becomes a foe for J'onn J'onnz after a blow on the head in Detective Comics #318, working with J'onn and Diane through the most of the rest of the Martian Manhunter's run in Detective Comics (until #326).
House Of Mystery 143
As J'onn had abandoned his identity as John Jones to a supposed death in his last issue in Detective Comics, he thought he could spent more time searching down the Idol-Head of Diabolu (a mystic item that causes unimaginable threats).
One of those threats was an enlarged and enraged Zook!
J'onn was able to stop his friend from hurting him (or himself), and Zook was restored back to normal by the end of the issue.
House Of Mystery 151
Zook doesn't appear again until House of Mystery #156, helps J'onn against his foe Professor Arnold Hugo in House of Mystery #157, helps end the menace of the Idol-Head Of Diabolu in House of Mystery #158, then pops up in #166, #168, #171 and #172 as Martian Manhunter contents with Mr. V and the criminal organization of Vulture, though isn't there when J'onn confronts their secret leader in his last solo feature of the 1960s in House of Mystery #173 (March-April, 1968).
Quite a few appearances for an "imaginary friend"....no matter what J'onn's dad says on the CW Supergirl!