  At the lowest masses, the distinction between brown dwarfs and giant
exoplanets is often blurred and literature classifications rarely reflect the
deuterium burning boundary. Atmospheric characterisation may reveal the extent
to which planetary formation pathways contribute to the population of very-low
mass brown dwarfs, by revealing if their abundance distributions differ from
those of the local field population or, in the case of companions, their
primary stars. The T8 dwarf Ross 458c is a possible planetary mass companion to
a pair of M dwarfs, and previous work suggests that it is cloudy. We here
present the results of the retrieval analysis of Ross 458c, using archival
spectroscopic data in the 1.0 to 2.4 micron range. We test a cloud free model
as well as a variety of cloudy models and find that the atmosphere of Ross 458c
is best described by a cloudy model (strongly preferred). The CH4/H2O is higher
than expected at 1.97 +0.13 -0.14. This value is challenging to understand in
terms of equilibrium chemistry and plausible C/O ratios. Comparisons to
thermochemical grid models suggest a C/O of ~ 1.35, if CH4 and H2O are quenched
at 2000 K, requiring vigorous mixing. We find a [C/H] ratio of +0.18, which
matches the metallicity of the primary system, suggesting that oxygen is
missing from the atmosphere. Even with extreme mixing, the implied C/O is well
beyond the typical stellar regime, suggesting a either non-stellar formation
pathway, or the sequestration of substantial quantities of oxygen via hitherto
unmodeled chemistry or condensation processes.
