What was going to happen at JFK? The passengers would have to sit down until the horses were unloaded? I smell nonsense....
Absolutely a possibility. I have experienced this situation (although rarely) with the combi. The passenger doors are left closed after parking until one or 2 pallets are off-loaded. This will sometimes have to be done when the dead load MAC is critical.
As far as the number of horses involved, KLM will put 3 horses side by side in a pallet-sized horse box. If the horses are small enough, sometimes a fourth horse is placed laterally behind them giving you 4 per box.
I have carried 20+ polo ponies from Buenos Aires on the combi this manner, without problem.