Parting tool or bedan - depending on the desired length of the tenon.
Take small steps, measure frequently with calipers. If the end objective is a tenon that must fit into a mortise, test frequently, and gradually reduce the diameter to the target. I often make pieces with 1/2 or 5/8" tenons that fit into drilled mortises, so I drill a test hole in a scrap using the same bit that will be used to create the actual mortise to use as a gauge.
Sometimes, if the tenon is to be longer (say greater than 1/2"), give it a gentle taper, and do a test fit with the mortise that it must fit into. As the tenon approaches the desired final diameter, it will slip partway into the mortise, leaving a burnished line at the point on the taper that has the exact diameter - that makes it easier to turn the rest of the tenon to the desired diameter. The tip of the tenon will be slightly undersized - that's not a problem, and in fact putting a small chamfer on the end of the tenon helps with final assembly.
Related subject - glueing a tenon into a mortise. Ideally, the tenon will be just a smidge undersized to allow room for glue. Use a scrap of wood (I sometimes use plastic coffee stirs) to smear glue inside the mortise rather than on the tenon - that way, surplus glue is forced into the mortise. Glue applied to the tenon tends to be forced out of the joint. But a problem is that if the mortise is blind (ie, just a hole drilled into timber), air can be trapped in the mortise when you try to insert the tenon, and forcing the tenon into the mortise also squeezes glue out of the joint, potentially leaving the joint weak. One solution to this is to drill a weep hole in the side or bottom of the mortise using a very small bit (less than 1/16") to provide a path for air to escape when you push the tenon in. That hole will fill itself with surplus glue, so it doesn't require any repair later. But another approach is to use a dremel to cut a groove along the length of the tenon to provide an escape path for air. Again, the groove will fill with glue and won't actually weaken the joint.