I do not think insurance fraud is involved in this particular fire, but it is not unheard of. It is nice that you think so highly of horse owners, and the overwhelming majority of them would never even consider killing a horse for money, burning or any other means, but not everyone is so honest, or humane:
It’s very expensive to be part of the horsey set boarding just one quarter horse, let alone doing it at scale. There’s also the perception of prestige, so it draws shady sorts from time to time just as any moneyed environment does. The circumstance here very likely is on the up and up, but the arsonist being connected to the stable owner is going to give rise to questions because such arson for profit is not unheard of, horrifying as it obviously is to anyone who knows horses at all.