Problems snubbing HF noise on flyback converter transistor
From what I observe in your waveforms, when the transistor turns off you are settling down to a peak voltage of about 60 volts and this is plainly not right for a flyback design running from 12 volts because the peak voltage seen (ignoring the initial glitches due to leakage inductance) should be about 24 volts i.e. about double 12 volts.

Because you are seeing about 60 volts this tells me that your transformer has the wrong turns ratio to suit the load on the secondary. Or, put another way, I think you are slipping into CCM (continuous conduction mode) due to the light load.

The glitch at the beginning of the 2nd half of the switching cycle could be so big due to the transformer problem I've alluded to. To resolve this any further you need to post your exact schematic with the exact values, operating frequency and transformer design.

Added section

The transformer is a 10:1 step up device producing an output of 430 volts and this means the flyback at the primary is 12 volts + 43 volts = 55 volts. OK you are seeing 60 volts so it's not too far off. You are probably generating a bit more than 430 volts.

The leakage inductance is 300 nH and this is what gives you the big pulse at the start of the 2nd half of the switching cycle. I would recommend a zener diode tied to the positive supply to catch this but you need to get the power rating sufficient AND you need to state what maximum output voltage you expect to produce because if it is 850 volts (as per your edited schematic), the natural flyback voltage will be 97 volts and you cannot do anything about that so you might consider zener clamping at 100 volts above 12 volts.

Anyway, with 169 pF of output capacitance when off and 300 nH of leakage inductance you will get somewhat distorted ringing at a frequency of 22 MHz and this doesn't look like a million miles away from what you are seeing.