This character is not known to have existed prior to the simplification of Chinese characters by the People's Republic of China in 1956.
The simplification of 衛 to 卫 may have been influenced by the Japanese katakana ヱ(we), cf. the Japanese ryakuji⿰彳ヱ.
In Zhongwen Da Cidian《中文大辭典》, a dictionary based on the Japanese Dai Kan-Wa Jiten and published in Taiwan from 1962 to 1968, 𠀄, a character which is visually similar to 卫 (乛 on top instead of 𠃌), is recorded as an unorthodox variant of 衛.
Another theory is that 卫 is a variant of 𫝀, which is the top component of 韋 in the middle of 衛.