I'm primarily referring to animals but a prehistoric plant, fungus, bacteria, or virus that qualifies for such a role would be cool. How would it go, how would they be stopped? Would they be predatory or vengeful, or just feral? Should it be a grounded encounter or supernatural? Animal horror can mix survival, political, and moral themes, and the possibilities with these are endless.
An ancient viral or floral spore contagian. Whether it be time travel to past or present or genetic engineering. A rampaging predatory Dromaeosaurid or Entelodont haunting a town, or a pseudo-kaiju scenario of a sauropod rampaging a suburburban area. What do you imagine?