I wonder... but if you consider how horribly long it can take to get the Draconack - Hungry avatar, that might put the likelihood of a Grarrl randomly eating anything into perspective?
I mean the mechanics of that avatar exists solely because any petpet in your inventory has the potential to eat food that's in there, doesn't it, but it is very rare that it happens from just normal site wanderings. I think it happens so rarely that people don't realise/forget that all petpets have this ability, not just the drackonack. I'm gonna assume the Grarrl's eating stuff from your inventory works along the same principle though. With regard to them eating an actual pet; with only one person reporting that happening that's firmly in the scary anomaly category right now, isn't it.
Asking the Goog - which incidentally felt the need to counter my question with:
Er, nooo...
I find out that as well as as Grarrls and Skeiths, Jetsams do the inventory scoffing too. Should have realised that was pretty likely!
Also, some artwork that pretty accurately reflects the actual size of Jetsams. Don't forget how we had collective brain-implosion when we found out about this. xDDD http://neocolours.me.uk/petsizes.php