I like the Ram idea if we're going with animals, or even a bull. Either is known for charging and if it's a Cavalry replacement (clearly Heavy being made of stone or metal) then I assume it is to go head to head with lances. Again I could go either way about a Dwarf riding it and part of me still wants a non-animal construct but a Rune inscribed stone/metal ram or bull would be leaps and bounds above a Dwarf riding a boar. Of course it's Aaron we've got to convince not me.