Is the floor covered in rubber? If so, lift it (if you can) & have a really good look at the floor, check for any bits that look soft/discoloured/rotten. If you can't lift the rubber you'll need to walk about & see if there are any bits that feel soft/springy. Then get some tarp & a torch, lie on you back & have a really good prod and poke about underneath. The floor ought to be 2 layers of wood (it's usually marine ply). I'd get any bits that look like they are deteriorating replaced, it's not worth the risk of your horse sticking its foot through. If you're not confident doing this yourself, find out who in your area maintains horseboxes & see if they'll check the floor for you.