If you have been to the Bungie home page and clicked on the Frontier link, you have probably seen the images of the different classes of the Fallen. These are the [url=http://images4.wikia.nocookie.net/__cb20130524153210/destiny2579/images/thumb/9/9f/Fallen_Dreg_Model.png/368px-Fallen_Dreg_Model.png]Dreg[/url], [url=http://images1.wikia.nocookie.net/__cb20130524153221/destiny2579/images/thumb/9/9b/Fallen_Vandal_Model.png/368px-Fallen_Vandal_Model.png]Vandal[/url] , and [url=http://i.imgur.com/eEr54m6.jpg]Captain[/url] classes of the Fallen. While not a class, we have seen a fourth type of Fallen in the [url=http://www.ign.com/videos/2013/06/14/destiny-behind-closed-doors-demo-e3-2013]E3 gameplay reveal[/url] named[url=http://www.destinypedia.com/images/c/cb/Destiny_E3_2013_Demo%2C_Rixis%2C_side_view.jpeg] Rixis, Archon Slayer.[/url]. As I have presented you with all of the different types of the Fallen we have seen so far, I go onto my observations. First is the possible role of genetic fitness in the Fallen chain of command. Looking at the Dregs, you can see that they are thinner than the other classes, have smaller horns on their heads, and only have two arms. The Vandals are a bit larger, have four arms and larger horns, and have better weapons than the Dregs. Finally, in terms of the base classes, the Fallen Captain is very large and possesses very large horns. In reference to Rixis, he has huge horns on the side of his head, is gigantic and, as we can see in the video, can do some type of space magic that lets him teleport. He is also named and given the title of Archon Slayer, showing his importance. My thought is that the role a Fallen plays in their House (The existence of multiple houses of the Fallen was revealed in[url=http://www.ausgamers.com/features/read/3321493] this [/url]interview) is in some way determined by their genetic fitness. Those who are strongest are trained the most and taught to be Captains, commanders of armies. Those who are average are trained to be basic soldiers, the Vandals. Finally, those who have genetic defects or are smaller are not overlooked. Every soul is needed to gain ground against the other Houses and humanity. So, the Dregs are allowed to serve and fill out the ranks. Rixis is an anomaly, an outlier of robust genetics. He is strong, giant and possess the ability to use magic, unlike the other Fallen. So those are my thoughts on the difference in appearance. What do you think? Any other ideas?
I think Rixis was just a Fallen Captain. Not a fourth class of any sort. But I do think your other ideas are interesting and I agree with you. The dreg just seem like basic trained foot soldiers. Maybe genetically modified to be thinner, smaller, and weaker for their role in the Fallen military. Or maybe just a common defect within their population. But who knows? This is all just speculation.