The Kaiju In Pacific Rim Explained
In Japanese, the word "kaiju" literally means "strange beast" and is used to refer to any giant monster movie, as well as the monsters themselves. Godzilla, King Kong, and yes, even Clifford the Big Red Dog can all be considered kaiju (via CBR). But while we can comfortably say that kaiju are something we never have to deal with, the people of the "Pacific Rim" universe aren't so lucky. For them, kaiju are as real as a hurricane or a tornado, and twice as deadly.
To repel these inhuman threats, the humans in "Pacific Rim" have developed Jaegers, a category of combat mecha so massive that they require two pilots to function. With Jaegers, humans had a weapon equal to a kaiju in terms of both size and destructive capabilities. Even so, kaiju inevitably still pose a major threat to the human characters. And even if they seem like familiar winks toward Godzilla to us, to the humans of "Pacific Rim," they are foreign in a way that demands explanation. So here are the kaiju in "Pacific Rim" explained.
There are five categories of kaiju
If you've seen any of the kaiju designs in "Pacific Rim," then you know that no two kaiju look exactly the same, or even remotely similar. While all of them take features from a variety of aquatic life, such as sharks or crabs, they all do so in a way entirely unique to them and that grants them unique abilities. The kaiju Otachi, for instance, has bat-like wings and can spray acid at its opponents. Meanwhile, Knifehead's swordfish-like appendage enables it to cut through Jaegers like butter.
For this reason, the Pan Pacific Defense Corps doesn't classify kaiju by appearance, but by measuring the levels of toxicity and displacement they create in the water when they appear. In other words, size and potential biological harm are more important than anything else. When these measurements are at their lowest, a kaiju is classified as Category I, with the upper limit resulting in a Category V. To date, only two true Category V kaiju have ever appeared in either the original "Pacific Rim” or its sequel, "Pacific Rim: Uprising" (via Pacific Rim Wiki).
Kaiju are just alien bioweapons
The reason the PPDC uses this odd category system instead of simply measuring a kaiju's threat level based on its abilities is because, technically speaking, all kaiju are the same. On a purely genetic level, each of them has the same exact DNA, and it's merely that different genes are expressed between specimens (via Pacific Rim Wiki). This makes it nearly impossible to predict what any one kaiju is capable of when they first appear. It also makes the kaiju even more mysterious, because usually, creatures with different features aren't genetically identical. This is the first indication humans receive that there is something behind the kaiju, creating and controlling them.
As it turns out, in "Pacific Rim" lore, the kaiju are simply biologically engineered weapons created by a race of extra-dimensional aliens known as the Precursors. These are the same aliens who opened the Breach, the dimensional portal in the Pacific Ocean through which all kaiju emerge. Using kaiju, the Precursors are able to conquer entire planets, wiping out the native population without ever visiting them. By sending out smaller Category I kaiju and then moving on to Category V and beyond, the Precursors are able to scout the selected planet and learn about it before sending in the most threatening monsters to clean up the vermin.
Because of this, kaiju are especially otherworldly
Though it might not be obvious at first, the kaiju's status as aliens is actually reflected in their designs, and we're not just talking visually. While the inspiration for the monsters' apperances was taken from Earth creatures, head creature designer for "Pacific Rim" Wayne Barlowe has stated that the kaiju in general aren't meant to evoke any of them (via Gizmodo). Instead, they were chosen on a basis of whether or not the creatures seemed intimidating. If a kaiju's resemblance to an Earth creature made it scarier, then that alone was enough to make it a contender for the final cut of the movie.
Similarly, the kaiju's personalities are also meant to reflect their alien nature. Aside from having a pure hive mind, the kaiju of "Pacific Rim" are simply single-minded killing machines. "I'm not so sure I want [humanness] in all my designs especially if I'm striving for pure 'alien-ness,'" Barlowe told Gizmodo. "In the case of the kaiju, you're right. They are monsters bent on apocalyptic destruction."