In trophoblast cells, major constituents of placenta, LTR retrotransposons including endogenous retroviruses (ERVs) are highly expressed. However, the roles and/or evolutionary significance of ERV-derived genes involved in placental formation, particularly structural diversity, have not been well characterized. In this review, we illustrate the placental formation and then list the diverse morphology of placental structure among mammalian species. We then describe the detailed history of ancient placenta development with the paternally expressed gene 10 (Peg10/Sirh1), Peg11/Sirh2, and Sirh7/Ldoc1, genes that are derived from LTR retrotransposons, followed by the capture of envelope genes derived from ERVs (hereinafter called ERV-env) such as Syncytin-1, -2, -A, -B, -Rum1, and BERV-K1/Fematrin-1. The latter ERV-env genes are responsible for trophoblast cell fusion, resulting in multinucleate syncytiotrophoblast cell formation, and possibly, the morphological diversity of placentas. In fact, multiple endogenization of retroviral genes has occurred independently in different mammalian lineages, and some ERV-env genes use the same transcription factors for their gene expressions. We therefore speculate that ERV-env gene variants integrated into mammalian genomes in a locus-specific manner replacing the genes previously responsible. In addition, ERVs also work as transcriptional regulators of various genes such as interferon-stimulated genes (ISGs). The previously proposed “baton pass” hypothesis therefore suggests that evolutionary events caused by multiple successive retrotransposon integrations, likely resulted in effective trophoblast cell fusion, ISGs' transcription in a temporal and spatial manner, and/or increased diversity of placental structures.