mcd is a device driver for a Mitsumi CD-ROM drive, models FX001, FX001 high speed, FX001D, or LU005, that is plugged into the Mitsumi controller card. It has major number 16.
Normally, this device driver is included in the kernel when you install or update COHERENT. To configure this driver, log in as the superuser root, and execute script /etc/conf/mcd/mkdev. Then run the command
/etc/conf/bin/idmkcoh -o coh.test
to build a test kernel that includes the driver.