Yes, Ive got one- mine could prob fit in 17.2 hand horses. Mine is the larger one tho, the smaller versions would juuuuust squeeze in a small 16.2 I think...If your lucky!
I've got the LT45 which is slightly longer than the LT35. My 16.1 fits in there with loads of room to spare but they do differ depending on the coachbuilders etc. With the 3.5t lorries you do have to be careful about the payload as you could easily go over the limit with 2 horses in.
you can up rate to 7.5 ton by up rateing the springs.
i know someone who got one as a 3.5 but it had a living. everytime i saw her go out in it i used to cringe because it sat so low at the back, the wheels tucked in the arches, this was only with the one horse in aswell.
in the end the spring and shocks went and she had to get new ones, she asked about puting the 4.5 or 5.5 ton ones on but was told not to bother. in the eyes of the garage it had the right spring for the right weight but as soon as you put a horse on it it was far to heavy.
the thing was i never had this problem on my transit (dont hate me) but i only had an ifor type body with no living.
i think you can do this to most boxes i.e change a 7.5 ton to say a 10 but you have to get them regraded.
As others have said, the size of horse they will take depends entirely on the box that has been built on the chassis. Mine has lots of floor space, but head height is 7ft (same as Ifor 505), so wouldn't take over 16hh I don't think. With me and all my stuff in mine I have 850kg spare for horses.