Kaiju translates from Japanese as "strange creature" or "monster" and is usually depicted causing havoc in a major metropolitan area. The most famous Kaiju is Godzilla, dated back to 1954. They recently starred in Guillermo Del Toro's 2013 movie Pacific Rim, fighting against Jaegers (giant robot fighters like the Megabot). The comeback of Godzilla is due for 2014.
The Kaiju is an immensely powerful card, with 7 power when attacking. Even without the attacking bonus, with 3 power it is still more than enough to combat the average unit.
Despite having one of the highest total attack powers in Villages, the Kaiju is not an unbeatable monster. His ability states that two units can combine their strengths to combat the Kaiju. Therefore cards such as the Wizard and the Ace become very useful. Kaiju is susceptible to the ability of the Joker as the Joker's ability causes the lowest power to win.
The Megabot card is designed to be the ultimate anti-Kaiju card as, in normal circumstances, it cannot lose against it (much like the relationship between the Knight and the Dragon). Therefore, if you must use the Kaiju to fight the Megabot, make sure to have a Pig or other power-increasing card handy.