  Studying the properties of young planetary systems can shed light on how the
dynamics and structure of planets evolve during their most formative years.
Recent K2 observations of nearby young clusters (10-800 Myr) have enabled the
discovery of such planetary systems. Here we report the discovery of a
Neptune-sized planet transiting an M4.5 dwarf (K2-25) in the Hyades cluster
(650-800 Myr). The lightcurve shows a strong periodic signal at 1.88 days,
which we attribute to spot coverage and rotation. We confirm the planet host is
a member of the Hyades by measuring the radial velocity of the system with the
high-resolution near-infrared spectrograph IGRINS. This enables us to calculate
a distance based on EPIC 210490365's kinematics and membership to the Hyades,
which in turn provides a stellar radius and mass to 5-10%, better than what is
currently possible for most Kepler M dwarfs (12-20%). We use the derived
stellar density as a prior on fitting the K2 transit photometry, which provides
weak constraints on eccentricity. Utilizing a combination of adaptive optics
imaging and high-resolution spectra we rule out the possibility that the signal
is due to a bound or background eclipsing binary, confirming the transits'
planetary origin. EPIC 210490365b has a radius ($3.43^{+0.95}_{-0.31}$R$_{E}$)
much larger than older Kepler planets with similar orbital periods (3.484 days)
and host-star masses (0.29$M_{\odot}$). This suggests that close-in planets
lose some of their atmospheres past the first few hundred Myr. Additional
transiting planets around the Hyades, Pleiades, and Praesepe clusters from K2
will help confirm if this planet is atypical or representative of other
close-in planets of similar age.
