This study discusses the behavior of resonant tunneling diode (RTD) oscillators when a transmission line (TL) stub is added. The TL stub acts as a delayed feedback unit, resulting in unstable and complex oscillation behavior. Circuit simulation showed that the circuits generate various waveforms, including chaos, by changing the stub length. Experimental demonstration of the simulation results was performed using circuits fabricated with hybrid integration techniques using an InGaAs/AlAs RTD. These complex signals have potential for various applications in the THz frequency range. On the other hand, this finding is significant for the design of THz oscillators using an RTD, since even a small metal pattern can cause such a feedback effect in the THz frequency range. In particular, interconnect wiring patterns can cause this effect because reflection due to impedance mismatch is unavoidable.