I encountered a yellow healthed enemy from a beacon mission in explore cosmodrome. It's the only time thiat has happened and it's the only time a beacon mission was at all life-threatening. I got it from the beacon just to the left of where you enter the Divide (the mission voiceover mentions an ambush), which is very close to the actual fight, so if it isn't there it may spawn at a nearby beacon. It's been almost a week since it happened so my memory is a little unreliable, but the boss had the same entrance method as the Archon boss (jumping out of a large dark pipe sticking out of the wall), and I think it must have been an Archon too. I haven't been able to get that beacon mission again, nor have I found anyone mentioning it online, so either it's rare, you only get it once and/or it wasn't actually the boss/mini-boss it seemed to be. I was very startled by the fight so my observational skills weren't at their peak. Please let me know if you know what's up. Edit - Based on the comments it is an actual Archon mini-boss other players have encountered.