MR and CP both have links to the racing community (I believe its ok for me to say this now because we are allowed to discuss them and this is at least general knowledge at this point) but KS does NOT. The only ties horse racing has to this case is that the two people last seen with KS are tied in with the industry. Racing is mostly irrelevant here, except that I believe it is relevant that CP and MR do not have steady jobs, which allows them to carry on the lifestyle they lead. I believe this is relevant for the case, because it means they had quite a lot of free time on their hands in the days before and after KS's disappearance.
Theoretically speaking, if one owned a particular horse which earned them a large amount of money in a short amount of time, this might lead them to create a Facebook account for said horse and make a series of humorous posts from said horse. But I don't believe something like this would be a factor worth discussing in this case.