I think maybe the knifeman has an animated weapon but I'm told animating the weapon bone can cause some freaky results.

The skeletons below simply tell the game what animation set to use for this unit. The example below means that his primary attack is a Spear animation set and his secondary is non_shield.
You can look up these animation sets by searching for MTW2_HR_Spear in data\descr_skeleton.txt All unit skeleton types are listed in here.
For example if you wanted him to play sword animations for his secondary, you would replace MTW2_HR_Non_Shield with -> MTW2_HR_Sword
If you wanted his primary to be mace animations you would replace MTW2_HR_Spear with -> MTW2_HR_Mace

Hope this helps :)

<Skeletons>
<Skeleton>
<Mount>Horse</Mount>
<Primary>MTW2_HR_Spear</Primary>
<Secondary>MTW2_HR_Non_Shield</Secondary>
<PrimaryAttachment>MTW2_HR_spear_Primary</PrimaryAttachment>
<SecondaryAttachment>MTW2_Sword_Primary</SecondaryAttachment>
</Skeleton>