Bitis gabonica rhinoceros
B. g. rhinoceros has a distinctive set of enlarged nasal scales that look like a pair of horns on its nose. This is a characteristic that it shares with a close relative, B. nascornis. However, B. nasicornis has a brighter color pattern and a narrower head. B. g. gabonica has no such enlarged nasal horns, and is overall somewhat smaller than B. g. rhinoceros. Also, with B. g. gabonica, the dark triangular marking leading back from the eye towards the angle of the mouth is divided. With B. g. rhinoceros it is not.
West Africa from Ghana west to Guinea, including Sierra Leone, Liberia and Ivory Coast. Togo is also mentioned.
One of my favorite snakes, was pretty big, id say 4ft at least. Saw at Houston zoo.