Milky Way halo substructures identified in dynamical space are known to suffer from contamination from the Milky Way in situ stars, which makes their accreted origins uncertain. We present detailed chemical abundances of 35 stars belonging to two sets of dynamically tagged groups, Rg8 and Rg9, to investigate their accreted nature. Both groups are composed of stars with low orbital energy and very retrograde orbits. We find that Rg8 and Rg9 are chemically indistinguishable across 20 elements, from C to Eu, strongly indicating that they belong to the same structure. The iron-abundance distribution of this low-E retrograde group has a prominent peak at [Fe/H] ≍ ─2.1, revealing that its main population is very metal-poor (VMP), and a secondary peak at [Fe/H] ≍ ─1.5, very likely due to contamination from Milky Way in situ stars. These groups also heavily overlap with the Thamnos substructure in dynamical space, and we thus use them to investigate the chemical properties of Thamnos. The dominant, low-metallicity population provides strong evidence for the ex situ origin of Thamnos, as does its VMP nature. We do not see any evidence of an α-knee in our sample, which is consistent with previous studies. Comparison with the Cetus-Palca stream in the chemical space shows similar abundance distributions, and thus it suggests that the Thamnos progenitor dwarf galaxy had a truncated star formation history due to its early merger with the Milky Way.