The couatl, A, is the weakest of the angelic beings. It is one of the few non-sea monsters to possess a drowning attack. This is usually irrelevant, but can be deadly if the couatl happens to be on a square containing water (it needs to be on the water, not just next to it). It will use its crushing attack when presented with an opportunity and being an Angelic being, it does not respect Elbereth.
The couatl is generated lawful, and is therefore friendly against other lawfuls.
A couatl might be generated to chase you when praying to an extremely angry god, or as a minion of the former god when converting altars.