Early on in Predator's special effects tests, a couple big changes were made. Firstly, the titular character's look was totally overhauled from a design that initially had a long head, big glowing eyes, and a woeful lack of dreadlocks. Secondly, they had to take Jean-Claude Van Damme out of the Predator suit, because it turns out he was in there, and he didn't understand what was happening nor why he didn't get to kickbox anything.
As explained in this video from the Stan Winston School, a young JCVD, just jump-kicked out of the plane from Brussels, was originally cast as Predator's Schwarzenegger hunter. That didn't last too long, though. Van Damme knew he was meant to be a star, to one day play Guile from Street Fighter, so he wasn't too happy to be hidden behind a bulky suit you couldn't even spread your legs 180-degrees in. Also, he didn't get that the bright red version of the costume was colored to make the Predator's invisibility effects possible in the green jungle. He just thought he was playing a big red bug, and he was rightfully concerned about that.
So, he eventually left the production, to be replaced by the vastly taller Kevin Peter Hall. But as you can see below, some footage of Van Damme running around in the costume still remains, a vermillion time capsule full of Belgian seething.