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Among the iron-based superconductors, FeSe has been extensively studied for exploring the high transition temperature (T_c) superconductivity, as well as the exotic physics of Majorana states hosted in topological superconductors. Here we use first-principles approaches to identify a new class of isovalent and structurally identical counterparts of FeSe, namely, the monolayered CoX (X = P, As, Sb, Bi), which are not only promising candidates for realizing high-T_c superconductivity, but also offer new opportunity to study topological superconductivity. We demonstrate that, similar to the recently reported CoSb monolayer, the CoX (X = P, As, Bi) monolayers can adopt the FeSe-like layered structures and possess superconducting properties comparable with or superior to FeSe and CoSb, mainly attributed to the electronically isovalency nature of these systems. More strikingly, our further analyses show that all these four CoX monolayers possess nontrivial topological properties characterized by band inversions, odd Z₂ invariants, and exhibition of topological edge states either in freestanding form or supported on SrTiO₃. These findings can provide new platforms for exploring topological superconductivity in two-dimensional limit.