The prolific Ti3Co5B2 structure type has produced exciting materials with tunable magnetic properties, ranging from soft magnetic Ti2FeRh5B2, to semihard magnetic Ti2FeRu4RhB2 and hard magnetic Sc2FeRu3Ir2B2. Density functional theory (DFT) was employed to investigate their spin–orbit coupling effect, spin exchange, and magnetic dipole–dipole interactions in order to understand their magnetic anisotropy and relate it to their various coercivities, with the objective of being able to predict new materials with large magnetic anisotropy. Our calculations show that the contribution of magnetic dipole–dipole interactions to the magnetocrystalline anisotropy energy (MAE) in Ti3Co5B2-type compounds is much weaker than the spin–orbit coupling effect, and Sc2FeRu3Ir2B2 has, by far, the largest MAE and strong intrachain and interchain Fe–Fe spin exchange coupling, thus confirming its hard magnetic properties. We then targeted materials containing the more earth-abundant and less expensive Co, instead of Rh, Ru or Ir, so that our study started with Ti3Co5B2, which we found to be nonmagnetic. In the next step, substitutions on the Ti sites in Ti3Co5B2 led to new potential quaternary phases with the general formula T2T′Co5B2 (T = Ti, Hf; T′ = Mn, Fe). For Hf2MnCo5B2, we found a large MAE (+0.96 meV/f.u.) but relatively weak interchain Mn–Mn spin exchange interactions, whereas for Hf2FeCo5B2, there is a relatively smaller MAE (+0.17 meV/f.u.) but strong Fe–Fe interchain and intrachain spin exchange interactions. Therefore, these two Co-rich phases are predicted to be new rare-earth-free, semihard to hard magnetic materials.