No, electrons are not the only negatively charged subatomic particles. While electrons are the most well-known negatively charged particles, there are other particles that also carry a negative charge. One example is the muon, which is similar to an electron but has a much larger mass. Another example is the tau particle, which is even more massive than the muon.
In addition to these charged leptons (electron, muon, and tau), there are also negatively charged mesons, which are composite particles made up of quarks. For example, the negatively charged pion (π-) is made up of a quark and an antiquark combination.
It's worth noting that within the Standard Model of particle physics, which is our current understanding of the fundamental particles and their interactions, only these known particles carry negative electric charge. However, there could be yet undiscovered particles with negative charge that are not yet accounted for in our current understanding.