I have a different suggestion for that high attack, low defense beastman. Toco researched deeply and discovered original Milton Bradley concept art for the Dwarf and Wizard expansion packs that were never released. I don't wanna step on his toes, so rather than grabbing his picture to post here, I'll provide this link to his site:
http://www.toco.be/ahq/unreleased/index.htmScroll towards the bottom and you'll find a Ratman holding a Battle Axe/Axe or Halberd. I'd say the Ratman has 2 or 3 Defend dice and 2, 3, or 4 Attack dice. One or two of the Skaven from Advanced Heroquest could work for the Ratman (or mod them):
These Marauder Skaven also fit the bill nicely:
Scrolling to the very last picture, you'll find the Centaur armed with a Battle Axe/Axe and a Shortsword. Looks like 2 or 3 Defend dice and 3 to to 4 Attack dice plus 2 Attack dice that could be combined for 5 to 6 Attack dice against a single, unfortunate opponent! It never made it to the prototype figure stage, but that's probably for cost-cutting reasons rather than appropriateness for inclusion in Hero Quest. Here is a cheap Mage Knight Centaur Lieutenant that has a single Battle Axe:
Brunor from Heroscape could be modded with an Axe, but he's big, probably 5+ Body big (compare to the dragon rider):
Here's a cheap D&D Miniatures Harbinger Centaur that would need some modding:
Confrontation has some Celt Centaurs:
Scratch-building could build you one from Warhammer parts. Check out this from Cool mini or not: