The short answer is we don't know for sure. We have some sources, but many of them are hearsay, and some are satire.
What we can say is that elite pankratiasts ate a lot, and many of them ate a lot of meat.
Athenaeus, in The Deipnosophists, Book X, on gluttony and drunkenness, gives examples of the diets of pankratiasts (and wrestlers and other athletes), including quotes from other writers:
Astyanax of Miletus: being once invited to supper by Ariobarzanes the Persian, when he had come, offered to eat everything that had been prepared for the whole party, and did eat it. ... And they say that the dinner which he ate by himself at Ariobarzanes' table had been prepared for nine persons.
Theophilos making fun of the diets of athletes in his comic play, The Pankratiast: Of boiled dishes there were nearly three pounds' weight. ... A snout, a ham, four pigs' feet. ... Three ox-feet, and a fowl. ... Two pounds of figs. ... A dozen half-pints of wine, neat.
Theagenes of Thasos, Olympic winner in many sports, including pankration: ate a bull single-handed, "And as I'd undertaken, I did eat a Maeonian bull. My own poor native land of Thasos could not have purveyed a meal sufficient for the hunger of Theagenes. I ate all I could get, then asked for more."
Different trainers and/or athletes (but not pankratiasts AFAIK) are credited with being the first to use high-meat diets:
Diogenes Laertius claims: Pythagoras is said to have been the first to train athletes on a meat diet. The first athlete he did this with was Eurymenes. Formerly, athletes trained on dried figs, moist cheese, and wheat
Pausanias wrote of Dromeus of Stymphalos (a runner): He is said to have first thought of eating meat
as part of his training diet. Until then, the food for athletes was cheese fresh out of the basket
For more, see:
Paul Christesen and Donald G. Kyle, A Companion to Sport and Spectacle in Greek and Roman Antiquity, Wiley, 2014
M. I. Finley, H. W. Pleket, The Olympic Games: The First Thousand Years, Courier, 2012
Louis E. Grivetti and Elizabeth A. Applegate, "From Olympia to Atlanta: A Cultural-Historical Perspective on Diet and Athletic Training", The Journal of Nutrition, Volume 127, Issue 5, 1 May 1997, Pages 860S–868S, https://doi.org/10.1093/jn/127.5.860S
Louis E. Grivetti, "Diet, Training, and Ergogenic Aids: Search for the Competitive Edge. The Evidence from Antiquity", presented at the VIIth IOC Olympic World Congress on Sport Sciences Physical, Nutritional, and Psychological Care of the Athlete in the 21st Century, Athens, 2003: https://nutritionalgeography.faculty.ucdavis.edu/wp-content/uploads/sites/106/2014/11/Olympic.IOC_.Presentation.AthensGreece.pdf
Michael B. Poliakoff, Combat Sports in the Ancient World: Competition, Violence, and Culture, Yale University Press, 1987