Unidentified hawk-moth caterpillar

I found this caterpillar already dead in water in the forest.

When I found it, it was pale green, about 6.5 cm long, with a long horn at the rear of the body.

I was only able to photograph it properly a few days later. By then, the body had darkened to an almost brown-black color.

I included the original field photo, several views on a light background, the underside, head, rear end, and a photo with a ruler.

It appears to be a hawk-moth caterpillar. The exact species has not been identified yet.