[quote=@Shayd]
It would be easy enough for the creature to be immune to its own poison. Many real-enough animals are. But someone else is going to have to explain the fire-breathing part. I'm not a biologist.
[/quote]

[quote=@Mardox]
Perhaps their insides are extremely flame-retardant and they have a special organ that produces gases that combined create a burst of flame? That's how it works with the dragons of my homeworld.
[/quote]

The only thing that I can see plausible for the fire-breathing is if they had teeth that could produce sparks (which I don't really know how it'd work or its effect on the creature) and they could breath out their farts (methane)