Perhaps the horse is Marcroix. It appears from this link that he won four gold medals, 3 of which had the same rider. If it's on the net, it must be true. ;-)

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Most gold medals (horse)

Marcroix was ridden by Charles Pahud de Mortanges in three of his four medal rounds, 1928-32.

http://www.djstexasstateofmind.homestead.com/EquestrianWorldRecords.html