That's not a bad looking piece of wood, but those holes have been drilled in there. It's most obvious with the one in the middle, where you can still see three separate drill holes placed like a triangle to make one big cavity.
What's left is an interestingly shaped end of a branch that's sticking up, and the base is probably attached by screws to a piece of flagstone or a heavy tile that's now below the substrate. This would be to make the whole contraption sink.
If you can find a weathered piece of wood that's still hard in the forest, making an aquarium decoration like this can be a nice Saturday afternoon project :thumb:
