  $A$Co$_2$Se$_2$ ($A$=K,Rb,Cs) is a homologue of the iron-based
superconductor, $A$Fe$_2$Se$_2$. From a comprehensive study of RbCo$_2$Se$_2$
via measurements of magnetization, transport, neutron diffraction,
angle-resolved photoemission spectroscopy, and first-principle calculations, we
identify a ferromagnetic order accompanied by an orbital-dependent
spin-splitting of the electronic dispersions. Furthermore, we identify the
ordered moment to be dominated by a $d_{x^2-y^2}$ flatband near the Fermi
level, which exhibits the largest spin splitting across the ferromagnetic
transition, suggesting an itinerant origin of the ferromagnetism. In the
broader context of the iron-based superconductors, we find this $d_{x^2-y^2}$
flatband to be a common feature in the band structures of both
iron-chalcogenides and iron-pnictides, accessible via heavy electron doping.
