From the performance side of things:
-Minis sweat less than large horses in the same conditions.
-They seem to oxygenate better as their heart rate and respiration are quite remarkable after covering the same distance as a horse literally four times their size.
-They don't seem to get the same sort of leg injuries as large horses, either from jumping or strenuous driving exercise. Possibly because of shorter pasterns not allowing for hyperextension, or because of lower body weight to bone density ratios?
-A typical horse competing in combined driving events will pull about 50% of their own weight and come in sweaty and tired. A mini pulls 100-150% of their own weight and still bounces across the finish line.
On the physical side of things:
-Minis seem to have exactly as much hair as their larger counterparts, just crammed into a much smaller space.
Makes for majorly fluffy manes and tails as well as those infamous fur suits!
-Same amount of teeth and similar size, just squished in a smaller mouth.
-Same placental thickness but because the foal does not stretch it out in the last months as a full-sized horse does it can still be quite thick at birth and cause problems.
-Reproductive equipment in general seems to be the last thing to downsize. One vet I talked to also felt that their hearts and lungs remained oversized, possibly contributing to their amazing stamina.
On Medication:
-Minis are very prone to bute toxicity.
-It is not safe to give them Quest (I think it was Quest??) dewormer as they are extremely sensitive to overdose and can die from it.
If I think of anything else I'll post again.
Leia