  For on-axis typical short gamma-ray bursts (sGRBs), the forward shock
emission is usually so bright that renders the identification of kilonovae
(also known as macronovae) in the early afterglow ($t<0.5$ d) phase rather
challenging. This is why previously no thermal-like kilonova component has been
identified at such early time except in the off-axis dim GRB 170817A associated
with GW170817. Here we report the identification of an unusual optical
radiation component in GRB 070809 at $t\sim 0.47$ d, thanks plausibly to the
very-weak/subdominant forward shock emission. The optical emission with a very
red spectrum is well in excess of the extrapolation of the X-ray emission that
is distinguished by an unusually hard spectrum, which is at odds with the
forward shock afterglow prediction but can be naturally interpreted as a
kilonova. Our finding supports the speculation that kilonovae are ubiquitous ,
and demonstrates the possibility of revealing the neutron star merger origin
with the early afterglow data of some typical sGRBs that take place well beyond
the sensitive radius of the advanced gravitational wave detectors and hence the
opportunity of organizing dedicated follow-up observations for events of
interest.
