Nidovirus endoribonucleases (NendoUs) include nonstructural protein 15 (nsp15) from coronaviruses and nsp11 from arteriviruses, both of which have been reported to participate in the viral replication process and in the evasion of the host immune system. Results from a previous study of coronaviruses SARS-CoV, HCoV-229E, and MHV nsp15 indicate that it mainly forms a functional hexamer, whereas nsp11 from the arterivirus PRRSV is a dimer. Here, we found that porcine Deltacoronavirus (PDCoV) nsp15 primarily exists as dimers and monomers in vitro. Biological experiments reveal that a PDCoV nsp15 mutant lacking the first 27 amino acids of the N-terminal domain (Asn-1–Asn-27) forms more monomers and displays decreased enzymatic activity, indicating that this region is important for its dimerization. Moreover, multiple sequence alignments and three-dimensional structural analysis indicated that the C-terminal region (His-251–Val-261) of PDCoV nsp15 is 10 amino acids shorter and forms a shorter loop than that formed by the equivalent sequence (Gln-259–Phe-279) of SARS-CoV nsp15. This result may explain why PDCoV nsp15 failed to form hexamers. We speculate that NendoUs may have originated from XendoU endoribonucleases (XendoUs) forming monomers in eukaryotic cells, that NendoU from arterivirus gained the ability to form dimers, and that the coronavirus variants then evolved the capacity to assemble into hexamers. We further propose that PDCoV nsp15 may be an intermediate in this evolutionary process. Our findings provide a theoretical basis for improving our understanding of NendoU evolution and offer useful clues for designing drugs and vaccines against nidoviruses.