D&D 4E 4ed creature types and subtypes

[Xethreau]

They also have 'animus' as well as animate. I think it has something to do with whether something has a soul or not, but I'm a little fuzzy on the whole thing.

For a point of comparison, Grell are magical beasts while Grick are not.

Part of me wonders if it's just an editing mistake.

I think it is intentional. It seems to me that a Beast is a non-humanioid creature who's existence, while possibly not something that occurs in the real world, is not dependant on magic, like the grick or the dire wolf. A magical beast would be a creature similar to a beast, but dependent on magic to exist, like the grell (levitation) or a dragon (elemental breath weapons.)

I can't say this applies to all situations, but this is the impression I get.

 

[Shroomy]

Ahah - the animus thing is just a typo in Thirdwizard's monster and more, the Boneclaw was listed as an animus, but Boneclaws are actually shadow animates

I am wonder what a shadow or wraith is.

Shadow animate (spirit, undead)?

 

[Xyl]

It seems likely that the "origin" types correspond to parts of the 4e cosmology:

Aberrant = hailing from the Far Realm
Elemental = hailing from the Elemental Chaos (including the Abyss)
Fey = hailing from the Feywild
Immortal = hailing from a Dominion of the Astral Sea
Natural = hailing from the natural world
Shadow = hailing from the Shadowfell

I believe you've hit the nail on the head. You can find a creature's type line by asking three questions:

Where (which plane) did this type of monster originate? If it's from the normal world, it's natural. If it's from somewhere else, it's the appropriate origin, even if the creature was born in the natural world.

What is the creature's basic body plan? If it's generally humanoid, it's a humanoid. If it's the equivalent of an animal on whatever plane it came from (which probably means it's not very intelligent, among other things) it's a beast. Otherwise it's a magical beast.

Is it part of a larger group of monster types with a common identity, such as undead or eladrin? If so, give it the appropriate subtype(s).