Back in my message board days, that dark and lonely time when flame wars were a kick, there were a slew of learned commentators trying to piece together all of DC Comics' history into a single, logical timeline. That way lies madness, I assure you, but it can be fun to investigate. For years, I heard about this elusive "Golden Age Martian Manhunter" called Roh Kar, as well as this or that random, never reprinted story involving Martians. I gave up on trying to work out all those details in that bygone time, so when a discussion broke out about Joe Certa possibly swiping the look of J'onn J'onzz, I kept my trap shut.
According to Wikipedia,
In Action Comics #16 (September 1939), the Zatara story is called "Terror from Saturn". A teleportation beam plucks Zatara from Earth to Saturn where he meets Porra who is the spitting image of the Martian Manhunter.Honestly, it isn't as though the Martian Manhunter's costume is particularly strong or original, and we've had green men from the red planet since the pulps that inspired adventure comic strips. When you get down to the conceptual core, J'onn J'onzz is just John Carter traveling in the opposite direction, heavily influenced by his pal Tars Tarkas.
However, Idol-Head regular LissBirds chimed in with a pair of nice scans from the Zatanna story, and by gosh, that really does look like J'onn J'onzz! His colors are reversed, so that the straps across his chest are blue, while his cape and trunks are orange/red. Rather than a circular belt buckle, its a neck clasp. Like Michael Netzer and Mike Sekowsky before him, there are blue gloves over his green hands, with boots to match. Curiously, like the primordial Batman, this alien action star has a holstered pistol, plus a knife and whip! What a find!
LissBirds' images sent me scurrying for more reference on the internet. The Martian Manhunter Fotolog had a side-by-side comparison of the two aliens. Even accounting for paper stock, J'onn J'onzz is much more brightly colored, with solid black trunks, and cavalier boots. Aside from the differences noted, the costumes are strikingly similar. However, you'll notice J'onzz's oversized head, somewhat flat nose, overbite, and pronounced brow. The Golden Age character is much more classically handsome than the bizarre Martian in his earliest appearances. Facially especially, but also in general build, the resemblance doesn't really come into play until the 1960s. By that point, Joe Certa had reach his third reworking of the Martian Manhunter, beginning with the skinny, Simpson-headed initial model, through a bulkier, thuggish look, and ending with the svelte, handsome leading Manhunter that most resembles the Zatara villain.
The Fotolog also provided scans of "Terror from Saturn" in its entirety, if rather small, so I hope copyright laws are a bit looser in Spain. The first thing revealed is that Zatara was a pimp who put Superman's powers to shame, and Fletcher Hanks' Stardust the Super Wizard wasn't created in a vacuum. The next thing is that, like J'onn J'onzz, this story is centered around stealing from Edgar Rice Burroughs (more directly, in fact.) The Saturnian Porra greets Zatara with his whip, but is easily trounced, and revealed to be the Prince of Saturn, like Jemm: Son of Saturn. Porra's father Ool even looks like Jemm's uncle Jogarr, and has a similar warlike disposition. Zatara punks them both, convincing them that invading Earth isn't in Saturn's best interest.
Considering the links made between Mars and Saturn in the John Ostrander Martian Manhunter series, these green-skinned Saturnians could go a long way toward explaining the disparities between Silver Age Martians and the "Natural Form" version introduced in the 1980s. The story is definitely an intriguing artifact I'd love to see explored further by competent hands. Still, the Saturnians exhibit no superhuman powers, and are only superficially comparable to the Manhunter from Mars. Unlike Roh Kar, I think it's safe to say the two projects were created independently from one another, and connected only by coincidence and common inspiration.