My mayor worry was that "arcane creature" could be interpreted as applying to every single wizard, sorcerer, bard, assassin, etc, including things like dragons. I'd say to be native to Arcane terrain, the creature should either be a somewhat direct result of the magic in that terrain (not just there because it was attracted by the leylines, but somehow created, summoned, or called by it), or the magic should be a somewhat direct result of the creature (though the level of magic needs to be significant. It shouldn't be enough that there's a few walls of fire, but if the creature creates its personal Mordor...).