They were living in the region around the five mouths of the River. Ganga where it meets the sea. The kings possessed a great army including a frightening army of Elephants. This made Alexander, the Great to flee , fearing the Bengali Might. I have earlier written on the fact that the success of Alexander in India was a Myth and he did not defeat Porus.
I have written a number of articles on these subjects. In an earlier article that Alexander's triumph over Porus and Alexander' subsequent return to Greece looked suspicious., as thte source one depends on heavily is Megasthanes's Indica of Chadra Gupta Maurya Empire. Megasthanes writes in detail about Chandra Gupta even intimate details. But the catch is Megasthanes never met Chandra Gupta Maurya.