You can only see fire hounds spawn on the second autumn. That world is only on day 16 on the first autumn, only normal hounds spawn and it only less then 2-4 hounds, so this is clearly the work of a griefer.

Ah didn't notice them, then again I have seen public bases burn down to a fire hound, was the correct day for one to appear, rare but not impossible to run into one at the end of autumn.

Take a closer look on the top left corner and you will see the person responsible for the fire.

Let me guess, fire hound spawned?