As part of developing my naval rules and the “fantasy aircraft carriers” of large ships, I wanted to look at the weights of the flying creatures listed as potential mounts in the rulebook. I found four entries with six creatures that can be ridden - the griffon, hippogriff, pegasus, and three sizes of roc (small, large, and giant). The only two with listed weights are the griffon (500 pounds) and hippogriff (1000 pounds), both of which are capable of flying with a 450 pound weight unencumbered, and a 900 pound weight maximum. The pegasus and rocs have no weight listed.
As a point of comparison, I used the horses, as those are animals bred in the real world for riding. Their limits are 200/400, 300/600, and 400/800 respectively for light, medium, and heavy horse. Researching online, the general limits for light horse weight seem to be 840 to 1200 pounds, for medium 1100 to 1300 pounds, and for heavy 1500 to 2200 pounds. By dividing each weight capacity by the weight of the minimum and maximum for each mount class, I came up with estimates for mount size based on rider weight. Essentially, my estimate is that a mount should weight 19.3 to 25.9 times the weight of its unencumbered carry weight. This would also be somewhat of a high estimate, because flying animals typically carry less weight than walking animals. However, since these are mythological and unnatural creatures, I’m willing to stick with those numbers for now.
Using those numbers, the minimum, maximum, and average (rounded to the next highest 100) weights (in pounds) for the various flying mounts are:
Griffon - minimum 1736, maximum 2331, average 2000
Hippogriff - minimum 1736, maximum 2331, average 2000
Pegasus - minimum 1158, maximum 1554, average 1400
Roc, Small - minimum 579, maximum 777, average 700
Roc, Large - minimum 2315, maximum 3107, average 2800
Roc, Giant - minimum 11576, maximum 15537, average 14000
Note that the griffon and hippogriff are both the size of somewhat above average heavy horse, the pegasus is a large medium horse or small heavy horse, the small roc is lighter than a horse, and the large and (especially) giant roc are huge.