One of the largest horn coral fossils known to have existed was the Siphonophrentis elongata (Rafenesque & Clifford, 1820). When you find a piece of one these fossils you know it. It would be amazing to find one almost fully intact at this size. Fossil was probably found in the Jeffersonville Limestone of Clark County, Indiana, USA. This creature would have existed in the Devonian Period.
Second image is of the side view and is probably about 20 cm long and 8 cm in diameter.
This fossil is part of large collection of fossils that make up a large fireplace. The fossils are from a variety of places. Unfortunately, the creators of the fireplace have passed away making it difficult to determine the exact location of where this specimen is from.