It needs to be something that can realistically wield a hammer, which Fenrir proponents forget. There is no way this guy carries around a hammer the size of a building just to drop it when he takes his most powerful form. There are many possible zoans that could use it in a hybrid form, including a wolf, but a wolf transformation primarily provides a boost to biting and scratching, which does not benefit a massive hammer user at all. Furthermore, Loki’s tattoos are of wings.
There are other possibilities from Norse mythology that make more sense.
Odin’s horse, Sleipnir, which was an eight legged horse that can fly. In this case, his Zoan transformation gives him even further increased size and flight, and his hybrid form would affect his legs rather than his arms, allowing him normal use of the hammer with multiple benefits.
A bear of some kind or another, perhaps even simply a bear. Bears were revered by the Vikings, Odin’s favorite animal and the chief/most powerful totem. The only known hero besides a deity to shapeshift in Norse mythology is Bjarki, who could turn into a bear. We notably do not have a bear zoan even though it is a strong option. A hybrid bear form would give a simple increase to strength and size to allow for melee weapon combat. Berserkers, which Loki seems to resemble, are supposed to channel the spirit of bears.
He could also have a mythical raven fruit, and the hybrid form would grant him flight, and potentially magical powers, while keeping the rest of his body the same.