In most condensed-matter systems, local and collective excitations remain decoupled due to their distinct energy scales. Here, we identify the coupled local-collective excitations in the triangular antiferromagnet ${\mathrm{NaErSe}}_2$, by combining neutron spectroscopy with a total angular momentum modeling. The low-lying crystalline electric field (CEF) doublets include a dipolar ${\mathrm{\Gamma }}_4$ ground state forming a stripe-$x$ order and a ${\mathrm{\Gamma }}_{5,6}$ excited state with dipole-octupole character. High-resolution spectra reveal emergent symmetry-selected dispersions where magnon branches from the ground state are replicated on higher ${\mathrm{\Gamma }}_4$ levels but couple with the ${\mathrm{\Gamma }}_{5,6}$ levels to form a distinct multipolar band. An applied magnetic field reconstructs the CEF wave functions and polarizes the system into a multipolar ferromagnet, further reshaping the spectra. The study demonstrates the emergent coupling of local and collective excitations driven by strong spin-orbit coupling and establishes ${\mathrm{NaErSe}}_2$ as a platform for field-tunable multipolar excitations in frustrated magnets.

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• Crystal-field excitations
• Magnetic anisotropy
• Multipole order
• Spin dynamics
• Stripes
• Hamiltonian systems
• Rare-earth magnetic materials
• Triangular lattice
• Angular momentum
• Spin
• Crystal-field theory
• Inelastic neutron scattering
• Neutron scattering
• Specific heat measurements
• Spin wave theory