While the fire breathing instead of lightning is a legit point of contention, the dragon on the mural is more likely than not to be the Nidhogg.
First because if there's some massive Jormungandr inspired serpent in the OP world it makes sense for a nordic figure to be fighting it, and second because Oda made such a big deal of the Nidhogg size and it being the biggest dragon that Oda is likely not going to introduce an even bigger western inspired dragon, although that wouldn't be the first lie in the series I guess