Origins
Origin Traditions
The source gives three origin traditions: a Zi-surname line descended from Weizi of Song in the Spring and Autumn period; the Oroqen Mahayier clan using Meng as a sinicized surname; and the Meng lineage of the hereditary native officials of Shunning Prefecture in Yunnan during the Ming and Qing periods, identified as Bulang. It does not state that these branches share a common genealogy.
Reference: 猛姓_猛姓的来源和历史_百家姓
Historical Records
Textual Attestations
The source says that the Zheng Tongzhi, Genealogical Treatise records the surname Meng, and that the Xing shi kao lue, citing the Lu shi, identifies it as descended from Weizi of Song, therefore belonging to the Zi surname. Regarding the Guangyun statement that it descended from Jin grandee Meng Huo, the source, citing the Zheng Tongzhi, says Guangyun is erroneous and that Meng Huo should be a mistake for Meng Zu. These are evidentiary opinions transmitted by the supplied source.
Reference: 猛姓_猛姓的来源和历史_百家姓