  We study the emergence of quasiparticles in Hund's metals with an
SU($M$)$\times$SU($N$)-symmetric Kondo impurity model carrying both spin and
orbital degrees of freedom. We show that the coupling of the impurity spin to
the conduction electrons can be \emph{ferromagnetic}, notably for hole-doped
iron pnictides. We derive the weak-coupling renormalization group (RG)
equations for arbitrary representations of SU($M$)$\times$SU($N$). A
ferromagnetic spin coupling results in a protracted RG flow, accounting for the
surprising particle-hole asymmetry that is observed in the iron-pnictide
systems. We establish the low coherence scale $T_{\rm K}$ which depends on the
filling through the impurity representation. We also discuss the temperature
dependence of the spin and orbital susceptibilities. Finally, we argue that
this mechanism explains the strong valence dependence of the coherence scale
observed in dilute transition-metal magnetic alloys.
