Simply put, if you have a fish bucket in your world, it changes if you leave and come back. The strangr part is that the item name remains identical to before you quit, but once you place it, it'll have changed into a new fish, seemingly in a pattern of sorts.

I started out with a TriggerFish, it changes into a plum-grey SunStreak, and then a grey-magenta SunStreak.

I remember this bug being marked as fixed in 1.7, but it seems to have come back in a weird way.