In the Stargate movie, and in the early seasons of Stargate SG-1, the bad guys are resplendent in animal-head helmets: Horus guards (from the movie) wore falcon-headed helmets, Apophis and his serpent guards from SG-1 make their first appearance in sinister snake helmets, and when SG-1 meets Heru-ur, their first new System Lord other than Apophis, his goons are wearing the falcon helmets again. However, after the first couple of seasons, the helmets disappear. Why?
Out of universe, I have a pretty simple explanation. As the later seasons introduce more and more Goa’uld System Lords, it would just not have been practical (or affordable?) to design and build unique helmets for each System Lord’s Jaffa. Thus, the vanishing helmets reflect real-world constraints.
However, I think I like my in-universe explanation better. Remember the scene from the original Star Wars where Luke rips off a stormtrooper helmet and exclaims, “I can’t see a thing in this helmet!”? Well, maybe the Goa’uld decided that SG-1 could regularly defeat whole armies of Jaffa because the Jaffa couldn’t see what they were aiming at. Of course, the Jaffa’s accuracy doesn’t seem to improve even after they start running around bare-headed, but perhaps the Goa’uld thought it was worth keeping up the experiment…