>>716010645
It only happens in a specific scenario and only triggers under certain conditions.
The only other scenario that does it is L'Arc where you have to fight against the frog horse Montjeu if you perform TOO well during the final race in the second year of the campaign.
Though not random, some career stories have you fight against a super juiced horse that is normally unbeatable due to plot but could beat them if you planned/trained well before then.