Old bone found in a lake

you need to put a quarter or a small ruler next to the bone when you take its picture.

It looks like a long bone from a fairly squat animal (small-would be like a marmot/ if its large hmmm)

Its old enough to hve looked like its been somewhat mineralized. Does it scratch glass or is it scratched? same thing with a piece of hard plastic)

If youve got a small piece, does it smell like burning organic if you hit it with a torch?

There are all kinds of resources in texts that show bone "Keys" for various animals
Id like to see its size before opining.

Well I wrote that the length is 15cm (6 inches) so it's not small at all, but it's short and it's thick, like from a heavy but not tall animal, because it looks like leg bone but I have no idea what kind of animal could it be.
I can't update new pic with some scale object because I'm not in the city where I found and left it in.

we like to visually compare with the actual measuring reference .
I resized it on screen and what I see could be something like an instep bone from a cervid, or even some pleistocene fauna footbone. I compared it with some mammals that were present in Europe and theres quite a few possibilities.

Based on size and shape (6" long and fairly robust) I would say it belonged to a small stocky animal. And since you found it in central Europe, one of the most common animals to fit the bill would be a wild pig or boar of some type. Possibly an immature one.

I googled some pictures of pig bones and it looks like it might be the upper arm (front leg) bone (Humerus).

In order to narrow it down it would be helpful to see it in perspective as Farmerman suggested. But it would really help to date it (even an approximate age would eliminate a lot of possibilities) and cross section it to see if it's a mammal bone or something else.

In a very general sense, there's a much higher probability that you would find a bone from a common animal than a rare one. And it's more likely that you would find something recent than something extremely old. Based on those two things alone and the visuals, I would guess Pig or Hog. But it's just a guess.