  Earth co-orbitals of the horseshoe type are interesting objects to study for
practical reasons. They are relatively easy to access from our planet and that
makes them attractive targets for sample return missions. Here, we show that
near-Earth asteroid (NEA) 2015 SO2 is a transient co-orbital to the Earth that
experiences a rather peculiar orbital evolution characterised by recurrent,
alternating horseshoe and quasi-satellite episodes. It is currently following a
horseshoe trajectory, the ninth asteroid known to do so. Besides moving inside
the 1:1 mean motion resonance with the Earth, it is subjected to a Kozai
resonance with the value of the argument of perihelion librating around 270
degrees. Contrary to other NEAs, asteroid 2015 SO2 may have remained in the
vicinity of Earth's co-orbital region for a few hundreds of thousands of years.
