This iron bottle has a brass stopper, and appears to be badly corroded. When opened, the magic of the bottle will dissipate, and it becomes a regular, corroded iron bottle. 

Succeeding a DC 12 History check tells a PC that bottles of this make have been used to trap creatures in an arcane form of suspension. If the character can read under common, they notice that the command word is etched around the neck of the bottle. 

Opening the stopper, (and reciting the command word) will release the creature the flask contains. The creature is friendly to you and your companions for 1 hour and obeys your commands for that duration. If you give no commands or give it a command that is likely to result in its death, it defends itself but otherwise takes no actions. At the end of the duration, the creature acts in accordance with its normal disposition and alignment.

Opening the stopper without the command word, or otherwise breaking the seal of the bottle, will release the creature the flask contains. The creature is angry, but otherwise acts in accordance with its normal disposition and alignment.

An identify spell reveals that a creature is inside the flask, but the only way to determine the type of creature is to open the flask.  The creature contained in the flask is at the discretion of the DM. 

Notes: Control