  We present deep high-resolution ($\sim$50 mas, 8 au) ALMA 0.88 and 1.3 mm
continuum observations of the LkCa 15 disk. The emission morphology shows an
inner cavity and three dust rings at both wavelengths, but with slightly
narrower rings at the longer wavelength. Along a faint ring at 42 au, we
identify two excess emission features at $\sim$10$\sigma$ significance at both
wavelengths: one as an unresolved clump and the other as an extended arc,
separated by roughly 120 degrees in azimuth. The clump is unlikely to be a
circumplanetary disk (CPD) as the emission peak shifts between the two
wavelengths even after accounting for orbital motion. Instead, the morphology
of the 42 au ring strongly resembles the characteristic horseshoe orbit
produced in planet--disk interaction models, where the clump and the arc trace
dust accumulation around Lagrangian points $L_{4}$ and $L_{5}$, respectively.
The shape of the 42 au ring, dust trapping in the outer adjacent ring, and the
coincidence of the horseshoe ring location with a gap in near-IR scattered
light, are all consistent with the scenario of planet sculpting, with the
planet likely having a mass between those of Neptune and Saturn. We do not
detect point-like emission associated with a CPD around the putative planet
location ($0.''27$ in projected separation from the central star at a position
angle of $\sim$60\degr), with upper limits of 70 and 33 $\mu$Jy at 0.88 and 1.3
mm, respectively, corresponding to dust mass upper limits of 0.02--0.03
$M_{\oplus}$.
