Godzilla (Gojira) - The King of Monsters, radioactive dinosaur. Special features: Atomic breath, advanced healing factor, can fight on land and in the sea.
http://geektyrant.com/wp-content/uploads/2009/08/godzilla001.jpg

Mothra - Giant moth
Special features: Uses mandible to bite opponents, can create gales with wings, can also emit energy beams and poisonous powder.
http://www.ugo.com/movies/godzilla-adversaries/images/mothra.jpg

Rodan - Airborne prehistoric reptile
Special features: Can fly at supersonic speeds, is resistant to Godzilla's atomic breath
http://upload.wikimedia.org/wikipedia/commons/8/86/Rodan_poster_detail.jpg

King Kong - Giant gorilla
Special features: Semi-human intelligence and great physical strength
http://scifipulse.net/wp-content/uploads/2009/07/KingKong_1933_01.jpg

Gamera - Massive flying turtle
Special features: Extremely resilient shell, eats and breathes fire, can fly and has large, sharp teeth.
http://thelass.files.wordpress.com/2009/02/gamera_hill.jpg

Anguirus - Radioactive dinosaur, similar to an Ankylosaurus. Godzilla's most trusted friend and ally.
Special features: Sharp teeth and claws, spikes all over body. Can move move fast and can burrow substantial distances.
http://deweydjb.files.wordpress.com/2008/06/anguirus.jpg

King Ghidorah - The King of Terror. A three-headed mutant dragon.
Special features: Can emit gravity beams, fire and bolts of electricity from different heads. Has golden armour, and can produce hurricane winds.
http://www.cinemastrikesback.com/news/new%20dailies/GodzillaandToho/ghidorah1-1024.jpg

There are others too!
http://en.wikipedia.org/wiki/List_of_kaiju

I like Mothra because he looks so cute!