The question of whether human imagination knows no boundaries or is, alternatively, constrained by conscious or unconscious cognitive templates is a key issue in defining human mind. We try here to address this extremely large question by focusing on one particular element of imaginary creations, the specific case of human-animal hybrids. Human-animal hybrids are common inhabitants of human imaginary spaces, being regularly encountered across numerous mythologies, as well as in modern popular culture. If human imagination was unconstrained, it would be expected that such hybrid creatures would display roughly half human and half animalistic features. Using several different popular science fiction and fantasy settings, we conducted an analysis of the morphological traits of human-animal hybrids, both anatomical and phenotypic. Surprisingly, we observed extremely high conservation of human morphological traits in human-animal hybrids, with a contrasting high use of phenotypic (“cosmetic”) alterations, and with highly stereotyped patterns of morphological alterations. While these alterations were independent of the setting considered, shape alterations were setting-dependent and used as a way to increase internal coherence. Finally, important gender differences were observed, as female human-animal hybrids retained significantly more human traits than males did, suggesting that conservation of female appearance may bear essential evolutionary importance. Taken together, these results demonstrate the existence of strong cognitive templates which frame and limit the expression of the capacity of human imagination, and unveil some of the psychological mechanisms which constrain the emergence of imaginary spaces.