If you want to use the actual AEM Forms product then yes, you will need an extra license. You will also need at least AEM 6.0 to run it although AEM 6.1 is recommended if you want to run the latest AEM Forms 6.1 FP1.
Bear in mind that they are licensed differently - AEM is licensed per instance and AEM Forms is licensed per CPU. Best talk to your local Adobe sales rep for details.