"Demon" is a very broad category, and tends to be used in different ways by different people. That's why West is asking what these are - he's not unfamiliar with this sort of thing, but has never encountered anything like these squid guys, before. This'll be clarified a little more in the page I'm working on, now.
Gremlins are often called demons by ordinary Ekwara folk, and are really the most commonly encountered "demon" the average person's ever going to encounter. Demon, for most people, just means "evil spirit". Gremlins are a particularly clever and nasty sort of spirit-fauna that has the ability to manifest in Ekwara physically, especially when lost things from Earth appear, or when someone uses electricity-based technology, though in this, they're inconsistent enough (they don't show up when guns go off... most of the time, etc) that it's assumed to be intelligent behavior on their part, and not an automatic thing. For their own reasons, they have it out to cause trouble for people trying to use certain technologies.
As nasty as they are, gremlins are still native to Ekwara's spirit world, while demons and djinn manifest in either that spirit world or the physical world from elsewhere entirely.