  Non-thermal pickup ions (PUIs) are created in the solar wind (SW) by
charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It
has long been theorized, but not directly observed, that PUIs should be
preferentially heated at quasi-perpendicular shocks compared to thermal SWIs.
We present in situ observations of interstellar hydrogen (H+) PUIs at an
interplanetary shock by the New Horizons' Solar Wind Around Pluto (SWAP)
instrument at ~34 au from the Sun. At this shock, H+ PUIs are only a few
percent of the total proton density but contain most of the internal particle
pressure. A gradual reduction in SW flow speed and simultaneous heating of H+
SWIs is observed ahead of the shock, suggesting an upstream energetic particle
pressure gradient. H+ SWIs lose ~85% of their energy flux across the shock and
H+ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream
of the shock, such that the energy flux of all H+ PUIs is approximately six
times that of H+ SWIs. We find that H+ PUIs, including their suprathermal tail,
contain almost half of the total downstream energy flux in the shock frame.
