It has had a chequered taxonomic history, having served as a wastebasket genus for a handful of mysticete species.

The genus Plesiocetus was originally coined for three mysticete species from Neogene marine deposits in the vicinity of Antwerp, Belgium: P. garopii, P. hupschii, and P.

hupschii and P. burtinii were later assigned to Plesiocetopsis, which was originally erected as a subgenus of Cetotherium[3]) Later, van Beneden noted the similarities of Plesiocetus with the fin whale, so he renamed Plesiocetus garopii into Balaenoptera musculoides, which is invalid under current ICZN rules.

At the same time, he also erected the new species P. brialmontii on the basis of fragmentary remains, while referring Cetotherium dubius to Plesiocetus.

[7][8] For its part, P. burtinii was referred to Aglaocetus, while P. brialmontii was declared a nomen dubium.