Originally the story went that this lynel called Zarukoshnik (Zaru for short) was taken in by the Yiga clan at a very young age and trained as their attempt at having a "living weapon", but things didn't exactly work out the bigger they got.

Because of the diet having an emphasis on mighty bananas, he's smaller compared to a regular lynel.

Maybe he has access to Sheikah/Yiga magics and/or tech that lets him disguise his lynel-ness, on top of the fire breathing and teleporting they naturally possess