This is one of those tricky ones, and I never would've known about it without the tip from user shadzane. Thanks!
Tusky is a very intelligent walrus who is sort of Aquaman's equivalent of Krypto. He debuted in the first episode of the Aquaman cartoon, "Menace of the Black Manta" (September 1967).
He first appeared in comics in Aquaman #36, which was also in September 1967.
And both the episode and the issue were written by Bob Haney. So which came first? Well, we've seen several times before that the lead time for animation can cause characters created for animation to jump over to comics before the animation airs. And beyond that, check out the cover to Aquaman #36.
It's the only issue to feature that banner at the top, so I'm pretty sure this issue was meant to promote the cartoon by featuring characters created for the cartoon. You'll see more of those next week.