Horse trailer issues?

When travelling in a horse trailer my horse loses his footing and slips over on his knees or falls backwards. Its really hard to explain but he like falls over. He regularly cuts the top of his rump. I was considering buying a horsebox anyway but do you think he would travel well in one or do you have any tips to... show more When travelling in a horse trailer my horse loses his footing and slips over on his knees or falls backwards. Its really hard to explain but he like falls over. He regularly cuts the top of his rump. I was considering buying a horsebox anyway but do you think he would travel well in one or do you have any tips to stop him slipping?
Update: we were driving at like 20miles and hour and he is definatly slipping cause i traveled in the trailer with him. the trailer has like striped rubber mats on the floor?
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