When performing an important effort, large veins on the surface swell, especially those on the neckline. This a natural reaction as blood cools better when veins are bulging. 

At rest, the average breathing of the horse is between 8 and 12 cycles per minute: one cycle corresponds to one inhalation AND one exhalation. 

If your horse doesn’t urinate like the others, its urine being brown or darker than usual, you should immediately warn your guide: it indicates that its urine contains blood from injured muscle fibre. 

You should always check that your horse isn’t injured when brushing him, especially on the girth path and withers location. You shouldn’t ride him before its injury is treated. 

It is important to check that your horse’s tendons are not swollen. A persistent swelling means that your horse is working too much or is having tendons issues. 

Especially while on a long walk, your horse should be offered water at every opportunity.
A horse refusing water when others are drinking is not a good sign.