  Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile
playground for the discovery and exploration of physical phenomena and new
physics. In this Letter, we report the observation of an anomalous thermal Hall
effect (THE) with \k{appa}_xy ~ 1x10^(-2) W K^(-1) m^(-1) in an insulating van
der Waals ferromagnet VI3. The thermal Hall signal persists in the absence of
an external magnetic field and flips sign upon the switching of the
magnetization. In combination with theoretical calculations, we show that VI3
exhibits a dual nature of the THE, i.e., dominated by topological magnons
hosted by the ferromagnetic honeycomb lattice at higher temperatures and by
phonons induced by the magnon-phonon coupling at lower temperatures. Our
results not only position VI3 as the first ferromagnetic system to investigate
both anomalous magnon and phonon THEs, but also render it as a potential
platform for spintronics/magnonics applications.
