If this animal can sleep for decades and left over food in it's gut can produce methane then it is possible that this creature could actually have the ability to produce a flame.
To my knowledge there is no record of a living animal ever being able to do this.
The fact that it's body reduces in size so much gives the indication that like the Electric eel of Manaus (Brasil) it's vital organs are in its head.
Hunters and explores in the Amazon that have come across this creature have commented on it's huge triangular, wicked looking head. I believe since this animal can go from fresh water to salt water, this creature has been seen as a sea serpent and in Europe has been associated with the look of the devil, the horns, the big eyes and the triangular head.