So a kaiju (怪獣) is any kind of big, terrifying, and seemingly undefeatable monster in fiction. The kind we're most familiar with, like Godzilla, are daikaiju (大怪獣) which specifically refers to the giant kind. They vary from being fairly intelligent to being animals, and tend to attack cities and things, leaving devastation behind.

This is a pretty loose definition, though. So...

King Kong. Huge gorilla and protector of Skull Island. Otherwise, not particularly fantastical.
Is King Kong a kaiju?
Yes
67.2%
No
6.3%
Maybe?
14.1%
No but those big monster things he fights are
12.5%
Poll ended at .
In the Jurassic Park movies, the T Rex is pretty much undefeatable, apparently fairly intelligent, and good at destroying things.
Is a Tyrannosaurus Rex a kaiju?
Yes
26.3%
No
28.1%
Maybe when it was wrecking San Diego?
45.6%
Poll ended at .
The Ghostbusters did actually defeat the Stay Puft Marshmallow Man, but it was still a ridiculously big monster which wrecked New York.
Is the Stay Puft Marshmallow Man a kaiju?
Yes
87.9%
No
4.5%
No, too cute
3%
No, too puffy
4.5%
Poll ended at .

In pretty much any version of the story, the Tripods from War of the Worlds are basically unstoppable eldrich horrors which wreck everything in their path until they all get sick and die.

They're mechanical, but so is Mechagodzilla. They're from space, but so is King Ghidora.

@InvaderXan

Although, you do have to wonder where giant robots in the Mazinger Z tradition fit into the ecosystem.

@truculentsheep Hmmm... not familiar with those either!