The harpoon brownble split from its ancestor the brownble. When competing for space in the mud the brownble would often kill the burysquid with its spike. Over time it figured out that it could eat what it killed and developed a teeth to chew up the meat. Not needing the bristles anymore it could use 4 of its legs to walk on. The all of the spikes on its legs disappeared except for a few around the foot so it can grip the ground like claws. The other 2 tentacles became pincer claws. The pincers were modified spikes which can pick at the meat it eats and bring it to the mouth. The mouth itself is much longer, and has be more like a trunk that face forward so better reach its feeding arms. It can sit above its head or between its legs or even face backwards. It is very versatile.
Its eyes too have improved to look for prey. They mostly face forward but can turn around to see in any direction if it needs to. Its brain has gotten bigger and smarter so it can hunt more effectively. This "tentacle gills" has grown slightly shorter to no get in the way. Since it has no bones it has to stay in the water and walk on the river bottom. It cannot support its weight without the buoyancy of water.