Behemoth? You mean Bahamut right? According to the neutral encounter FAQ, they are found found at the highlands in Aurura Plains II.
For others:
Hydra
Wentinus II, Marsh
Ahzi Dahaka
Mt. Keryoleth II, Forests
Tiamat
Aurua Plains I, Forests
Flarebrass
Aurua Plains II, Highlands
The venerable thunder dragon can't be found in the wild.