Transmission spectroscopy is enabling precise measurements of atmospheric H2O abundances for numerous giant exoplanets. Constraints on hot Jupiter H2O abundances, and hence metallicities, have provided a powerful avenue to probe their formation. However, metallicity measurements for Neptune-mass exoplanets are only now becoming viable. Unlike hot Jupiters, exo-Neptunes are expected to possess super-solar metallicities due to accretion of H2O-rich and solid-rich planetesimals under the core-accretion paradigm. I present results from an extensive atmospheric retrieval analysis of the exo-Neptune HAT-P-26b. By simultaneously retrieving ground and space-based transmission spectra, the most precise atmospheric metallicity of an exo-Neptune to date is obtained. This analysis additionally reveals suggestive signatures of a combination of metal hydrides (at > 4σ confidence) which, if confirmed, represents the first inference of metal hydrides in an exoplanet atmosphere. Finally, the atmosphere and formation of HAT-P-26b is placed in context with the wider population of well-characterised planetary atmospheres.