What is this fossilized claw found in a Belgian forest? It is 14.5 cm long and 2.5 cm in diameter.
Looks like the proximal end of a rib of a large mammal (cow, horse, deer). Without information about how large it is, it is not possible to get closer to an identification. It's not a claw and very likely not a fossil. It just looks old and dirty.
Here's a horse rib cage (from https://www.rodnikkel.com/content/saddle-tree-blog-from-shop-and-desk/the-rib-cage/):
Ribs in the 4-7 range seem to be a good match for shape but not as much for roundness.