I don't think Prestwich is reliable on this identification - he wrote:
"The count of Chalons//, Peter, known as the Oxherd, invited the English
king to join him in a tournament, which took the form of Edward's men
against all-comers". As I noted before, "Chalons" for Chalon is a
give-away that Prestwich had not troubled himself to know much about the
matter.
Pierre le Bouvier and Peter the Oxherd are the same man. As I have
pointed out, he was not a count and his family's surname was Chalon not
Chalons. There were no counts of Chalons, that is in Champagne. In 1273
there was no-one going by the title count of Chalon, that belonged to
the duke of Burgundy from 1237 - the duke at that time was Robert II,
who was titular king of Thessalonica.
Pierre and his younger half-brother Jean of Arlay were not counts. Otho
IV, count palatine of Burgundy, the head of their family, was somewhat
too important a figure to be publicly humiliated by Edward without
mention in local sources.
In my view the likeliest person to be mistakenly named as count of
Chalon was Pierre's elder full-brother Jean, whose second wife was one
of the three heiresses of her grandfather Hugues IV, duke of Burgundy
(died 1272). Through her Jean was count of Auxerre - I suppose he may
have held Chalon as well, but if so he was not known by this as a
comital title.
Peter Stewart