Can anyone tell me what may have caused this

[Poe1]

Do you think this could affect my other fish

[SimonW]

It appeared to be ulcer caused by bacterial infection. Normally this happens when the water quality is low, mostly high level of ammonia, nitrite, very high level of nitrate, and some times other unusual toxic compounds.

But it is not sure that it was the ulcer that killed the koi. How long had the koi had this ulcer before it died? Have you monitored the water quality?

[fly4koi]

Need water parameter first, as other had suggested.