In recent years, two entirely new classes of planetesimals have been discovered in the solar system: interstellar interlopers and dark comets. These still-enigmatic objects are challenging our understanding of the behavior and properties of comets and asteroids. In this talk, I will review what has been learned to date from the known interstellar objects and dark comets, highlighting the attributes that are difficult to reconcile with previous models of planetesimal behavior. I will present recent hypotheses that can explain their unusual behavior through natural mechanisms, including the acceleration of `Oumuamua via release of radiolytically produced and entrapped molecular hydrogen. One of the dark comet candidates, 1998 KY26, is already the target for the extended Hayabusa 2 mission and exhibits favorable viewing geometry before 2025. The forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) is poised to further transform our understanding of these classes of objects, and I will discuss the feasibility of future discoveries via ground-based observations as well as possible intercept missions.