Many years ago, a carpenter built a low exterior wall out of 6x6 pressure treated lumber and used these fasteners to keep them aligned. They look like mini-railroad spikes, but I can't find them in hardware store websites.

I'm removing the rotted top piece, and I hope/assume the fastener is only stuck in the piece below it, rather than through multiple ones. Anyway, it's stuck pretty tightly and I'd like to identify it.

Also, any suggestions for removing it. The wood on top is so soft, I can't get much leverage if I try to pry it up.