A few weeks ago, I mentioned how murky the Animal type is here: http://skirmisher.com/node/1541 Now I want to look at the type with the most overlap with other types- Magical Beasts. Magical Beasts have overlap with Animals, Aberrations, Dragons and Vermin. I already explained Animals, so here are the other three:
I wouldn't even bother mentioning Aberrations but for three examples. Ropers are obviously Aberrations as they are very alien creatures, yet in the SRD they are listed as Magical Beasts. Is this just for mechanical benefits (Magical Beasts have a better BaB, hit die and saves) or did they consider the roper something like a marine worm? I am thinking the former but why not examine the latter? There are plenty of strange animals in the real world, mostly invertebrates, that could be either (or Vermin). Worms alone have many variations. I had a giant version of the arrow worm in Creatures of the Wastelands- they have eyes, fins and complex mouths. There are worms with tentacles, fangs, armor, eyes and "paddles". A few marine worms use tentacles to build tubes out of sand- squint hard enough and you might see a roper. Another invert is the rotifer, a vase shaped animal commonly found in freshwater. Their unique feature is a mouth surrounded by hairs that allow them to draw in and filter smaller creatures out of the water. It could easily be made into a Magical Beast or Aberration that inhales prey on land or in the water. In fact, one could argue that carrion crawlers are distant cousins- the hairs evolved into tentacles the same time the animal gained legs. Another rotifer could feed not on flesh, but rather metal and stone. Is it a Magical Beast or an Aberration? Rust monsters are Aberrations, but that doesn't mean much considering how grey the lines are. As for another example, the remorhaz is also rather alien and yet is a Magical Beast. It is a worm with legs (those actually exist, though they are not called worms by zoologists), fins, eyes and the ability to melt steel from a gut enzyme. The last example is the naga. It is an intelligent snake with some spellcasting abilities and yet is an Aberration rather than a Magical Beast or Dragon. Why aren't they Dragons?
Dragons vs Magical Beasts seems fairly clear cut, but then there is the behir. An intelligent reptile that spits lightning. That it has multiple legs isn't that much of an issue nor is the lack of an ability to cast spells (there are plenty of Dragons that are not "true" dragons that do not). Again, it is probably due to mechanical benefits rather than thought of what is a behir and why it isn't a Dragon. Between behir and ambush drakes (3.5 MM III, which are Dragons but could be Magical Beasts), the line is almost as blurry as the division between Magical Beast and Aberration.
As for Vermin, this is easier because Vermin automatically are mindless. Of course they do have minds (many inverts have very simple personalities and have a wider array of actions than you might think), it is just easier to call them that so they can have their immunity to enchantments and some illusions. But what are intelligent insectoids? They have been listed as Magical Beasts, Aberrations, Vermin (with racial traits that give them intelligence) and even Monstrous Humanoids (I don't get that one).
I find this frustrating considering how important monster types can be. Magic (spells and magic items), feats, class features and such that rely on monster types require more clarity in the division between the types (not to mention monster and setting design). In many cases I have no idea where the line is because it is so wide and grey. I have no suggestions, just observations.