All that colts that we have raised have had 2 testicles present at birth, and in the Minis most of those colts have not had them "disappear", which surprises me.
However, we have bought several colts, as weanlings, and I cannot say if theirs were present at birth or not...of the two that we bred with, one had his 2nd one drop when he was 18 mos. old, the other one was just past 2 (about 25 mos.) when the 2nd one dropped...each of these horses has sired one colt for us; I know that's not much of a number, but both of those colts had both present at birth & they have not gone up & out of reach. So it isn't ALWAYS a case of like father like son. I do agree, though, that true cryptorchidism is often hereditary, and in many cases a stallion that is late to drop will sire late dropping foals. Maybe that's usually true, but not always...
One colt that we bought still did not have any testicles down at a full 2 years of age. At that point we were starting to worry that we were going to have problems getting this fellow gelded--there were a couple times when I thought I felt a tiny little "pebble", and I was thinking maybe if we were lucky they were actually there, just very, very small for some reason. Well, one day just a week past his 2nd birthday Silver had one drop down--one very large, well developed one! The 2nd one, equally large & well developed, made its appearance about a month later. Gelding him was easy after all!
Up to the age of 18 months I don't worry too much--after all, it's common enough even for big horse colts to not drop until they're 18 mos. old. After that, though, I do get concerned, even though more than one breeder has told me it's "common" for Minis to not drop the 2nd one until their 3 year old year. I don't want to wait that long, thank you!