  We report the discovery of the transient ultraluminous X-ray source (ULX)
CXOU J122602.3+125951 (hereafter M86 tULX-1), located 3'52" (19 kpc) northwest
of the centre of the giant elliptical galaxy M86 (NGC 4406) in the Virgo
Cluster. The spectrum of M86 tULX-1 can be fit by a power-law plus
multicolour-disc model with a 1.0 [+0.8 -2.6] index and an 0.66 [+0.17 -0.11]
keV inner-disc temperature, or by a power law with a 1.86 +/- 0.10 index. For
an isotropically emitting source at the distance of M86, the luminosity based
on the superposition of spectral models is (5 +/- 1) x 10^39 erg/s. Its
relatively hard spectrum places M86 tULX-1 in a hitherto unpopulated region in
the luminosity-disc temperature diagram, between other ULXs and the
(sub-Eddington) black-hole X-ray binaries. We discovered M86 tULX-1 in an
archival 148-ks 2013 July Chandra observation, and it was not detected in a
20-ks 2016 May Chandra observation, meaning it faded by a factor of at least 30
in three years. Based on our analysis of deep optical imaging of M86, it is
probably not located in a globular cluster. It is the brightest ULX found in an
old field environment unaffected by recent galaxy interaction. We conclude that
M86 tULX-1 may be a stellar-mass black hole of ~30 - 100 M_Sun with a low-mass
giant companion, or a transitional object in a state between the normal
stellar-mass black holes and the ultraluminous state.
