Unless you are redefining the art-definition you only need to tell the game what art definition to use for this particular model. The only other things you would need to include is the changes to the file you are creating. For example your above mount, doesn't need anything except the supported unit types.
To have compatibility to the female henchmen mod you need to include
<SupportedUnitModelType>HenchmanFemale</SupportedUnitModelType>
But in my mod I already did that for the ravenous harriden mount. So really all you need for your mod is the following
<GameItemType InternalName="Mount_MythicTameRavenousHarridan">
<DisplayName>Tame Ravenous Harridan</DisplayName>
<Description>A tame Ravenous Harridan. Tame as can be. It is enchanted with calmness and speaks your language.</Description>
<Type>Mount</Type>
<CanBeEquipped>1</CanBeEquipped>
<CustomizationPointCost>0</CustomizationPointCost>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>UnlockCombatAbility</Attribute>
<StrVal>TangledWeb</StrVal>
<Provides>Tangled Web- Target enemy is immobilized for 3 rounds.</Provides>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_ChanceToAvoidProne</StrVal>
<Value>100</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_CombatSpeed</StrVal>
<Value>2</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_DodgeAgainstRanged</StrVal>
<Value>15</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Moves</StrVal>
<Value>2</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_WeightCapacity</StrVal>
<Value>25</Value>
</GameModifier>
<CutSceneDataPack>SpiderMountCutScenePack</CutSceneDataPack>
<IsAvailableForSovereignCustomization>1</IsAvailableForSovereignCustomization>
<IsAvailableForUnitDesign>0</IsAvailableForUnitDesign>
<AIData AIPersonality="AI_General">
<AIPrefType>AIPrefType_MOUNT</AIPrefType>
</AIData>
<ArtDef>Mount_RavenousHarridan_ArtDef</ArtDef>
</GameItemType>
Since the art definition is already defined it should work just the way you intend.