I created a FreeDos USB-Stick using Rufus and a spare PC and im now trying to boot to it from an Mac but the Stick isn't shown at the boot device selection screen. Can somone please help me?
Like all (fairly) modern Apple computers, they are confined to the UEFI boot process and are unable to boot with a legacy BIOS process.
FreeDOS is unable to boot in UEFI mode (see the FreeDOS wiki) and thus can not boot on an iMac.
You can boot FreeDOS on iMac, I've successfuly tested that on iMac from 2011. Although very unstable it actually worked.
iMac comes with UEFI what is stated in previous negative answer, but also supports so called legacy mode with BIOS emulation, this is actually how Bootcamp works to help you launch Windows on Macs.
You cannot easily though install FreeDOS with standard Bootcamp. What I found usable is to install customized UEFI preboot app called rEFInd (https://www.rodsbooks.com/refind/) which does the job to support other x86/x64 systems on Macs not only Windows, this way you can install freedos and linux machines from usb stick or cd provided to the computer.