Bog snakes were nasty, venomous snake Rahi. They lived in the muddy shallows off Onu-Metru and fed on small amphibians.1, 2 Onu-Matoran walking along the shoreline learned to be extremely careful so that they did not disturb a bog snake nest.2
Notes
The giant, scorpion-like Nui-Jaga liked to feast on bog snakes. The Nui-Jaga was very large, but the bog snakes were small and numerous. When a hungry Nui-Jaga came to a bog snake nest, the bog snakes came in from all directions – behind, up-tree, down-tree – at once. This confused the Nui-Jaga, as there were too many bog snakes for even its stinger to stop.3
Out-of-universe
Bog snakes were the same thing as Kuna, which is a term that LEGO became unable to use.4