A fish. Fossilisation requires very specific set of conditions - the creature in question has to have hard parts to be preserved, then it must die and end up somewhere where it won't be eaten or disturbed, it must be Anoxic so it won't decompose normally, it must be buried.
The fish has a bony skeleton, lives in the ocean. So, it may die, sink to the anoxic bottom part of an ocean/lake, be covered by sediment and maybe become a fossil, if nothing disturbs it.
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