Controlling a spark plug using 555 IC
80% duty cycle is much too high for an ignition coil. As vini_i said you are probably saturating the coil. If I remember correctly, most ignition coils have a primary coil inductance of around 8 mH. This means that its reactance at 5 Hz is about 0.25 ohms. This is extremely low. At 5 Hz with 80% duty cycle this means your 12 V source will be trying to source (12V/(0.25Ω+0.5Ω))*0.8 = 12.8 amps on average to the coil alone. No wonder your MOSFET is getting hot!

Your MOSFET definitely needs a large heatsink.

The second circuit appears to be using a snubber circuit across the MOSFET to protect against voltage spikes. However, I have never seen a snubber circuit with a diode (or two paralleled diodes) in series with the rest of the snubber. I think that would defeat the purpose and not allow current  to flow when it needs to. I recommend removing the diodes in the snubber branch.