While museums are considered to present authoritative representations of natural and cultural history, it is widely accepted that no display neutrally presents an objectively realized exterior world. In this piece, we further that argument by drawing attention to the narrative techniques implicit in staging extinct life, focussing in particular on the similarity between museum display and the tropes of fantasy worldbuilding. We present three short case studies in which Mesozoic life is used in narratives that are straightforwardly at odds with the scientific consensus: the Creation Museum in Kentucky, USA; Biddulph Grange in Staffordshire in the UK; and the display contexts of the dinosauroid, a speculative Stenonychosaurus model created by the Canadian Dale Russell. Our aim is to demonstrate how museums put genre and storytelling to counterfactual purposes. Museums, we conclude, build worlds: worlds that are putatively similar to the one we live in but can just as easily be fictitious. The fact/fantasy boundary is almost always more porous than our shared impressions of museum authenticity typically suggest.