  Type Ibn supernovae (SNe Ibn) are thought to be the core-collapse explosions
of massive stars whose ejecta interact with He-rich circumstellar material
(CSM). We report the discovery of a SN Ibn, with the longest rise-time ever
observed, OGLE-2014-SN-131. We discuss the potential powering mechanisms and
the progenitor nature of this peculiar stripped-envelope (SE),
circumstellar-interacting SN. Optical photometry and spectroscopy were obtained
with multiple telescopes including VLT, NTT, and GROND. We compare light curves
and spectra with those of other known SNe Ibn and Ibc. CSM velocities are
derived from the spectral analysis. The SN light curve is modeled under
different assumptions about its powering mechanism (${^{56}}$Ni decay,
CSM-interaction, magnetar) in order to estimate the SN progenitor parameters.
OGLE-2014-SN-131 spectroscopically resembles SNe Ibn such as SN 2010al. Its
peak luminosity and post-peak colors are also similar to those of other SNe
Ibn. However, it shows an unprecedentedly long rise-time and a much broader
light curve compared to other SNe Ibn. Its bolometric light curve can be
reproduced by magnetar and CSM-interaction models, but not by a
${^{56}}$Ni-decay powering model. To explain the unusually long rise-time, the
broad light curve, the light curve decline, and the spectra characterized by
narrow emission lines, we favor a powering mechanism where the SN ejecta are
interacting with a dense CSM. The progenitor of OGLE-2014-SN-131 was likely a
Wolf-Rayet star with a mass greater than that of a typical SN Ibn progenitor,
which expelled the CSM that the SN is interacting with.
