My horse becomes a lame and his hock catches when he walks

My horse is ten years old and healthy most of time. The case is very surprising and strange that he was healthy last night before he became a lame the next morning, and his hock catches when it walks.
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[dressage_rider_in_Oregon]

Sounds like it is possibly upward fixation of the patella.  He could have gotten cast in his stall over night and strained or loosened a ligament.  Not surprising or strange at all, as horses do get cast in their stalls or paddocks and injure themselves.  OR, he could have OCD in the stifle, and again something happened overnight.  OR he could have a meniscus tear.  It could be so many things.  Best to consult a lameness specialist.