Science fiction and fantasy cosplay has changed a lot in the last 35 years. Back in the late 1970s, there weren't very many rules, if any. And there certainly was nothing like a separate costuming fandom. There were also an awful lot of naked people (which eventually resulted in the only rule I ever remember seeing posted: "No costume is no costume"), making authentic Edgar Rice Burroughs characters fairly popular subjects.
Here are a few choice examples of 1970s cosplay, from my personal photo collection. Warning: Many of these are NSFW.
One thing I noticed in going through the slides — mostly taken at Worldcon masquerades and a few other cons — is the great sense of whimsy that permeated SF costuming decades ago. This is something that seems to be missing, now that costuming is taken so much more seriously.
I recognize a lot pros in some of those old pictures, too — such as Elfquest creator Wendy Pini as one of her own characters.
Oh yeah — I remember another rule. Food was nixed for costumes after one joker covered himself entirely in peanut butter (he was a turd literally and figuratively), which eventually turned rancid after 3 or 4 hours under hot lights. Oh yes, indeedy, those were the days...
Elfquest's Wendy Pini is on the right
Want to see more cosplay photos from 1977? Here's a second gallery: