Drifting in Desert is the first mission from Metal Slug 2. At the end of it you will encounter either The Keesi as a boss (Metal Slug 2) or an Iron Nokana instead (Metal Slug X).
You battle Arabians in Metal Slug 2 and Rebels in X, however the gameplay is pretty much similar. Just kill the infantry and advance to get your Camel Slug. Right before the camel, you must battle against the Minaret towers. If you managed to find him (he is hang quite high right before the towers and can't be seen from the ground) you will be assisted by Hyakutaro Ichimonji. After you ride the camel, you will be attacked by Masknells. Take care to hit the red one. Then advance to the end of the stage to fight the boss.
Metal Slug :
Absolute Thick Forest | A Wish For a Morning Glow | Iron Cavalrymen in Hell | Death Valley | The City Under Despotism | The Memorial Sea