The Monoceros Ring is a vast, low-latitude stellar overdensity wrapping roughly around the outer Milky Way disk, with stars on nearly circular orbits at galactocentric radii of order 15–20 kpc and a total stellar mass of about 108 solar masses, whose origin is debated between a warped/flared disk response and debris from a disrupted dwarf galaxy (Yanny et al. 2003; Ibata et al. 2003). In a standard ΛCDM, thin disks should be relatively stable and outer circular streams this massive and coherent are difficult to reconcile simultaneously with their near-disk kinematics, chemical patterns, and large spatial extent without fine-tuned merger histories or specific satellite interactions (Peñarrubia et al. 2005; Laporte et al. 2018).