A burnt-out car was found in the debris of the livery stables at the farm in Shadoxhurst, near Ashford, Kent. At least 12 other horses were saved, but four had to be treated by a vet for burns.
The cost of damage caused by the fire is estimated at pounds 100,000.
The farm has been run by Brian and Nan Fraser for just over five years, but it has been in Mr Fraser's family for more than 50 years.
His mother Nora, who lives nearby, said: "I can't believe anyone could do something as terrible as this. The fire totally destroyed the farm buildings. You can't help thinking of all those poor animals in the building. It's sickening."
Mr Fraser's brother, Tony, said a car had been driven through a fence and then into a barn where it was set on fire. He said the animals did not have a chance: "It is an horrific and disgusting thing to do. It appears to have been a deliberate attack, but I have no idea who could do such a thing or why."
The fire was discovered early yesterday and Kent Police confirmed that a burnt-out car was found. Detective Inspector Gerald Smith said: "You can imagine the grief and misery that has been caused by this appalling tragedy to each and every owner. Obviously we are treating this as suspicious."
John Painter, 48, of Shadoxhurst, whose wife, Julie, 38, kept a horse at the stables was shocked by news of the blaze. Mrs Painter's horse, Byuk, an Arabian gelding, survived, but Mr Painter said: "It is a very friendly operation - a sort of do-it-yourself livery stables.
"Owners buy their own feed and straw and the horses are looked after and bedded down in the barns. This is a tragedy for Brian and Nan and very upsetting for everyone concerned."
A spokesman for Kent fire service said: "The cause of the fire is being looked into by our investigation team."Reuse content