One night, I did a string of Garuda Extremes to get my entire group Garuda ponies (for real). One person already had it, so we were only going for seven. It took us 30 runs to get 7 ponies. So, 7/30 = 0.23, or 23%. If I had to guess off of my experience alone, the droprate is either 20% or 25%. It may be less than that. Possibly 10-20% for other sample sizes as large as ours. However, as cited above... some people go for dozens of runs and don't see a single pony. So, it's all a mystery unless we truly add up a huuuuuuuge sample size, but I don't think anyone is dedicated enough to document that.