Kaiju literally translates to "strange beast".
They are a race of creatures featured in Kinnikuman. They are distinct from Chojin and humans, and feature heavily earlier in the series, in which Kinnikuman they were often the 'creature of the week'. Kinnikuman would encounter a new monster in each chapter, and would usually defeat them and learn a lesson. The two main arcs that feature these creatures are 1st Kaiju Extermination Arc and 2nd Kaiju Extermination Arc.
The Kaiju are often not humanoid in appearance, and nor do they look like various objects or natural formations (like other types of chojin). They are often monstrous in appearance and less intelligent, usually controlled by chojin such as Kinnikuman or Kinkotsuman.
They rarely appear in more recent arcs.
Some Kaiju include: