For ancient Olympic combat athletes (pankratiasts, wrestlers, boxers):
Note that these athletes appear to have been the most likely to have special diets. Since there were no weight classes, size was a bigger advantage than in other Olympic sports. Combat athletes - called at the time the "heavy athletes" - were famed for eating a lot.
However, other athletes did have specialised diets. A diet with more calories and more meat appears to have been common. From the above discussion,
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.
Our information on their diet is rather sketchy. I don't believe that we have enough detail to reconstruct the evolution of the athletic diet. The Greeks believed that the athletic diet had evolved - the writers above explicitly wrote about this evolution (but only in simple terms: so-and-so invented the meat diet).
For more, see the references from the above discussion:
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