I don't know where you got the idea that active skills are called active just because active is the antonym of passive. Absolutely incorrect. In fact, passive skills didn't even exist as a legitimate type until way, way after the active skill set was firmly established. So to say that the original type was coined after the latter was typified doesn't make sense at all.
The devourer skill is one of the most controversial, because it pretty much is an active skill, except for two glaring details: you cannot remove the devourer passive skill, and it is innate to the creature (retained despite mutant status). The original argument in Elements regarding creature skills was that creatures could not have more than one skill. Devourer was considered an exception for a long while until Zanz commented, saying that Devourer's passive skill could not be an active skill, because then it would not have the Burrow skill. He then stated that creatures could not have more than one active skill, and that he gave Devourer its passive skill as a way for a creature to hold more than one skill at a time. A few months later, Zanz came up with more passive skills and a way to identify which creatures had which passive skills.
Scavenger, Phoenix, Light, Fire, Air, Earth, Venom, and Vampire are all non-innate skills that are removable by lobotomizing effects. They are active in the sense that they are trigger-ready, caused by an event (creature death, creature action, successful attack, etc.). Mummy's passive skill actually used to be an active skill, until Zanz made it a passive skill in order to be lobo-proof.
Primary, main, principal, major, or exposed all hint that you could possibly activate a creature's passive skill instead of its active. Passive means that you can't touch it - it's just there and it will happen. Active means that something is required for the skill to happen.