( The 'th' in this name is pronounced like 'th' in 'with' and 'n' like 'n' in 'one'.)
Jayathsena was one among the warriors who fought on the paandavas side in the great war. He was mentioned to be brother of Sahadeva, the Magadha king.
Also, there was one more Jayathsena on the kouravas side mentioned twice in Udyoga parva by Bheshma and Arjuna. He was also mentioned to be from Magadha province. Maybe, there were two Jayathsenas from Magadha, one standing on the paandavas side and the other standing on the kouravas side.
Also this was the private and secret name of Nakula during their secret stay at the city of Viraata.
Similar or same names for different people is a very common confusing factor in Mahaabhaaratham. We should be careful n relating the names to the persons and identifying them correctly.
"After praying to Naaraayana, Nara, Sarasvathi and Vyaasa, then one should read the Jayam."
DEDICATED TO MY DEAR NIECE VYSHNAVI KRISHNA