Fixed is boring. Possibly a reduced amount - but its FUN to try to get one and then baby it to level it up. They have a tendency to howl and run straight into a spear.
Maybe increase the pain in the ass level - which I don't really approve of - by making them more frail, increasing the magic resistance/resistance to beastlord. All of which just make it cause more reload, which also is not fun.
Maybe +3, and +1 per 3 levels. So a level 9 great wolf would tack on +6 attack (vs +9) and a level 12 would tack on +7 (vs +12).