The Otter Prisoner can be found hiding in the Church within the East zone.
The prisoner explains where the Toad People come from, and says that the Toads recently invaded their city and began to hunt the Otter People for food. They them self became a prisoner but managed to escape to safety by breaking their bindings with a rock.
Lore and Story Analysis
Extract from the Monoliths found in the Eastern region, see more here.
The Toad race invaded the Lake, following their leader, The Emperor. Their leader promised power and freedom in exchange for their obedience and devotion.
So, the Toads attacked and enslaved the Otter People living in the ruined city of the East zone. Enslaving many, killing many more. Those who've been to the Eastern zone know of the piles of corpses littered around, or thrown in the water of the lake.
By killing the Otters, the Toads have upset the balance and doomed the peace that could have arisen in the East. However, a few Otters remain hiding out in the church.