all 3 forms of lavos are lavos to some extent. but the lavos core is the essence of lavos - it is the culmination of genetic material into the best possible form that lavos could create for itself. that would explain why the central bit (and the inner form) are humanoid - humans are the most dominant and successful species on the planet, so lavos would be greatly (but not entirely) influenced by humans.
id also argue that lavos was the humanoid central bit, but he transferred his "essence" or mind into the right bit as a deceptive defense.
as far as the origin of lavos, who knows. the outer shell would probably be useful in space travel and burrowing into planets, but everything else is very open-ended.