Can anybody recognize this cardiac waveform?
To be conclusive, another couple of leads recorded at the same time as this event would be helpful though the fact that you do have a recognisable QRS complex and a T wave and that the QRS happens before you would expect it to based on the prior rhythm and the P wave, if that is one, prior to it is to close to the QRS complex to have caused it would suggest a premature supraventricular arrhythmia.  There are many technical texts to consult to refine this further, if this is a small animal ECG, I recommend Larry Tilley's textbook entitled ECG for the small animal practitioner.