I would say so depending on the build and whether the horse can cope with the headroom It's meant to take 2x 16.2 so I would have thought you might get away with it
My horse is 16.3hh and he travels fine in our 505, he is comfy even with another horse in. His ears do touch the ceiling though and fold over at the top! He doesnt mind!
Think it deends on the horse though, my friends 16hh refused point blank to go in her 505,she had to get a 510 whch it happily loaded in.
I am collecting a new horse tommorow and my lorry is refusing to start at the moment so may have to borrow the above trailer. The owner siad he is very quiet to load and travel and has been in a trailer before.
My 16.2 ID (very chunky tho) doesnt fit in a 505 at all, and when we tried she scraped/cut all down both sides of her stomach and bum (which admittly is big!!) coming out due to the angle and tightness. Also I would bear in mind, one bad travelling experience coudl put him off for life!
I have an Ifor Williams HB505 and Ali is 16.1hh. I travel him without partitions (as he seriously hates partitions) and I have to say the headspace is fairly tight.