Okay two things.
First, most monsters have no support for items. You can equip them but they won't appear visually. Specifically, they need a special skeleton which has points where items are equipped. What you are looking for is this:
<SkeletonPath>Gfx\HKB\Units\K_Male_Skeleton_01.hkb</SkeletonPath>
Any monster that has that can be turned into a champion for you to equip gear. Other monsters won't display any items (or rarely will - some support a main-hand weapon only).
Second, in order to equip various bows, armor and mounts you'll need something called a modeltype. A modeltype is a sort of "category" for what kind of stuff the unit wears. You'll notice in-game that kingdom leather armor looks different from empire leather armor. Females also wear different versions than males do. All that is because of modeltypes. But monsters do not have those modeltypes by default. And if a unit does not have a modeltype, or has a modeltype not supported by the particular item (and the item doesn't support non-specific modeltypes like melee weapons), the item is instantly terminated.
You'll have to decide for yourself what bodytype you wish to add for your monster. Modeltypes look like this
<UnitModelType>UrxenMale</UnitModelType>
and you can see more examples in CoreUnits.xml. It is also possible to add your own modeltypes, although that is more advanced and unless you have a specific purpose for it I recommend against it.
