Beorn was never anywhere NEAR the Pelennor Fields.
1) Beorn was a full-grown man in The Hobbit. By the time the story STARTS in Fellowship, Bilbo (who was 51 in The Hobbit) is having his eleventy-first birthday (for the uninitiated, that's 111). So, at the beginning of Fellowship, it has been 60 years since Beorn fought at the battle of Five Armies. (Annotation: Bilbo's speech in the Chapter "A Long Expected Party" in The Fellowship of the Ring.) Then, 17 years later Frodo FINALLY starts the trek towards Mordor with the One Ring. (Annotation: The Tale of Years, Appendix B, Return of the King). It is now 77 years since the battle of Five Armies. I don't know this for certain, as I haven't read the books in a while, but I believe I can safely say that Beorn is long dead by the time of the Battle of the Pelennor Fields.
2) Even IF Beorn was still alive, he would not have been anywhere near Pelennor. The North had troubles of it's own during the War of the Ring. Thranduil in Mirkwood was assaulted by the forces of Dol Guldur, including spiders, Lorien was attacked by hosts from Moria, and another host of orcs and trolls assaulted the lands of Lord Brand of Dale and Dain Ironfoot of Erebor. Beorn, assuming he was alive, would have been too busy defending his woodland home to go join the fight near Gondor.
So, no, there is no way that Beorn was ever anywhere near the Battle of the Pelennor Fields.