Rattlor is about as much of a Horde member as Kobra Khan is one of Skeletor's goons, meaning none at all. Rattlor and Kobra Khan are Snake-Men no matter what. The only way I can see Rattlor being part of the Horde is if he switched sides due to how frightening and powerful Hordak was and knew that staying with Hiss would be on the losing side.
As for Dragonblaster Skeletor's bio, it fits nicely. A lame bio for a lame figure.
Strongly agree, both of those guys are snakemen. I remember seeing it somwhere (i think in the comic books) that Tung Lashor and Rattlor said to Hiss that they only went to the Horde because the Snakemen were trapped in another dimension, something on those lines, i have pretty much everything in boxes at the moment so i cant look for it.