WARNING: Before using the dd command, make sure the target device is the USB pendrive to avoid accidentally wipe out one of your hard disks.

You can use the command dmesg to check the device name after you plug in the USB pendrive.

dd if=ubuntu-16.04-desktop-amd64.iso  of=/dev/sdi  ibs=20M  obs=20M

70+1 records in
70+1 records out
1485881344 bytes (1.5 GB) copied, 277.745 s, 5.3 MB/s

blkid /dev/sdi

/dev/sdi: UUID="2016-04-20-22-29-52-00" LABEL="Ubuntu 16.04 LTS amd64" \
TYPE="iso9660" PTUUID="0e0e8e70" PTTYPE="dos"

NOTE: The same method can be used to create a Debian bootable USB Pendrive