How do you get treeko in Pokemon Heart Gold?

Answer 1

Get all 16 gym badges, defeat Red, and then go back and talk to Steven Stone after you get the lost item (who should now be at Silph Co. in Saffron City), and he'll ask you what stone you would like (aka a Starter Pokemon from Hoenn). Since you want a Treecko, you would pick the Green Stone. Otherwise, you can pick the Red Stone for Torchic, or the Blue Stone for Mudkip.