You can easily prepare any ISO file to be booted from an USB thumb drive, by following the simple steps below:

1. Download UNetBootin from http://unetbootin.sourceforge.net/
2. Connect a USB drive that can be deleted during this process. Make sure, no important data is currently on this stick!
3. Start UNetBootin – no need to install
4. Select „DiskImage“ and „Type ISO“
5. Choose the downloaded ISO file of ESXi (or any other installation media on .iso) here.
6. Go for „USB Drive“ as „Type“  and double check, that you have selected the right drive letter here.
7. Press „Okay“ and wait for the process to be done.
8. Use this USB thumb drive to boot your installation media from.

The user interface of uNetbootin