Understanding and controlling the interactions between quasiparticles are central to condensed matter research. In magnetically ordered systems, the collective excitations of spin precessions are spin waves—the quanta of which are magnons. Despite the growing interest in manipulating magnonic states beyond thermodynamic equilibrium, elucidating and inducing coherent couplings between distinct magnon modes remain longstanding challenges. In this talk, I will discuss our recent work on coherent nonlinear magnon-magnon interactions in antiferromagnets driven by tailored terahertz fields. We have demonstrated a unidirectional magnon upconversion process and identified correlated magnonic responses at both the sum and difference frequencies of the interacting magnon modes. By tuning the difference frequency generation to match the energy of the low-frequency magnon mode, we achieved parametric amplification of magnon coherence. Furthermore, we reveal magnon self-interactions by increasing the driving fields to excite the magnon mode far from equilibrium. These findings offer valuable insights into coherent magnon-magnon interactions in antiferromagnets, advancing the fields of spintronics and magnonics into the ultrafast nonlinear regime.