Silk Worm

1988, developed by Tecmo

Silkworm is a classic horizontally-scrolling shoot-em-up. The main difference between this game and many other side-scrolling shoot-em-ups is that you will be on land as well as in the air. The first player would play as a helicopter, but if a second player joins the game, they would play as a jeep. Both players would have to protect each other as well as themselves. After a certain number of kills, a "goose" helicopter would form on the screen. If the players managed to destroy it they would get a weapon's powerup. This would go on throughout the entire game, and a "goose" could appear even during the fight with the end-of-level baddie.