You mount partitions. Not devices.
The name of the partition is /dev/<device type> < number> <partition>
If the pen drive has a single MS-DOS (FAT) MBR partition, a virtual disklabel will assign this partition as "i". The output of
# disklabel sd1
will show its partitions.
If this shows a partition "i", you would mount this with:
# mount -t msdos /dev/sd1i /mnt
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