If people are getting confused, just tell them to stop worrying about what the corpse was. You only see that AFTER the fight, where it doesn't matter what slayer you use anyways. EM's have been dressing monsters up as other monsters for years. It keeps things interesting.So according to Promathia the skin produces the slayer and that's what the code is linked to, I can accept that. But whats confusing people is the corpses they produce, the one we killed in Britain was a crimson dragon corpse at least it was titled as much, used those spells, and definitely had that loot. That's why people are confused.
Besides, if you were to do a "Demon" invasion, and every time you could only use the same exact type of monsters, that would be quite boring. So EM's can take, a Crimson Dragon for example which has newer abilities and an interesting move set, and make it a demon to fight.