How effective are these guys? And could they somehow be considered the predecessor to the modern tank just as the horses of the medieval age were?
Not so much the predecessor of the modern tank as the predecessor of the technical*, used in the same environments for much the same purpose. It carries the largest gun you can get into the area in the fastest possible way. Effective against infantry but not against heavy fortifications.
It's a desert specialist unit, used as mobile artillery in places you wouldn't be able to get heavy artillery to.
The zamburak became a deadly weapon in the 18th century. The Pashtuns used it to deadly effect in the Battle of Gulnabad, routing a numerically superior imperial Safavid army. The zamburak was also used successfully in Nader's Campaigns, when the shah and military genius Nader Shah utilized a zamburak corps in conjunction with a regular artillery corps of conventional cannon to devastating effect in numerous battles such as at the Battle of Damghan (1729), the Battle of Yeghevārd, and the Battle of Karnal.
A zamburak consisted of a soldier on a camel with a mounted swivel gun (a small falconet), which was hinged on a metal fork-rest protruding from the saddle of the animal. In order to fire the cannon, the camel would be put on its knees. The name is derived from the Persian word for wasp zambur (زنبور), possibly in reference to the sound earlier camel-mounted crossbows made. The mobility of the camel combined with the flexibility and heavy firepower of the swivel gun made for an intimidating military unit, although the accuracy and range of the cannon was rather low. The light cannon was also not particularly useful against heavy fortifications. - Wikipedia
* A pickup with a machine gun on the back.
Pack animals have been used to transport weapons for a long time. Of course the pack saddles were not designed for firing from the horse or mule, the gun was supposed to dismount.
For that matter, look at this image of elephant-carried guns in Abyssinia.