This is 12" of classic MechaGodzilla! This is the Millenium MechaGodzilla 3 aka Kiryu, from the movies Godzilla Against Mechagodzilla and Godzilla Tokyo SOS.
Introduced in Godzilla vs. Mechagodzilla (1974). He is Godzilla's mechanical doppelgänger and one of the most popular Toho kaiju. Mechagodzilla is also recognized as one of Godzilla's most powerful enemies (all iterations have at one point or another come very close to killing the King of the Monsters).
A great action figure.