Thanks for showing that code (I don't actually have the DLC so couldn't fix anything here before).... Here is the fix put it in the mods folder run mods and it is fixed... https://dl.dropboxusercontent.com/u/75549875/Fallen%20Enchantress/DLC04_HorseFix/DLC04_HorseFix.xml
You are completely correct about the problem there. In order to overwrite the game item types you need to include everything about the item since it is an overwrite not a combine effect (like abilities)
Thus, they would have to include the entire code from the original horse for this to work appropriately. From https://forums.elementalgame.com/434780/page/1
GameItemType - I'm starting with these because they are the basis of what I'd expect. Whenever I create a mod for one of these types that already exist, all tags that were previously defined in the old type are deleted. So, if there is a sword I can easily overwrite it with my own definition. Except... if I overwrite all the items, the system that arranges random loot (using <Likelihood>) completely breaks down from monsters (not goodiehuts).
Thus, to fix this problem you just need only add this into their downloadable content.
<GameItemType InternalName="BasicHorseMount">
<DisplayName>Horse</DisplayName>
<Description>Horses grant your units more speed in combat and in travel and make them immune to being knocked prone.</Description>
<Type>Mount</Type>
<CanBeEquipped>1</CanBeEquipped>
<AdditionalTrainingTurns>20</AdditionalTrainingTurns>
<ShopValue>120</ShopValue>
<ProductionRequirement>
<Type>Resource</Type>
<Attribute>Horses</Attribute>
<Value>1</Value>
</ProductionRequirement>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Attack_Boost</StrVal>
<Value>2</Value>
<Provides>+2 Attack for the first turn of combat</Provides>
<xActions>1</xActions>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_CombatSpeed</StrVal>
<Value>2</Value>
<Provides>+2 Initiative for the first turn of combat</Provides>
<xActions>1</xActions>
</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_Moves</StrVal>
<Value>2</Value>
</GameModifier>
<CutSceneDataPack>HorseMountCutScenePack</CutSceneDataPack>
<IsAvailableForSovereignCustomization>0</IsAvailableForSovereignCustomization>
<Prereq>
<Type>RestrictedAbilityBonusOption</Type>
<Attribute>DLC04_UndeadFaction</Attribute>
<Target>Player</Target>
</Prereq>
<Prereq>
<Type>RestrictedAbilityBonusOption</Type>
<Attribute>LegacyOfSerrane</Attribute>
<Target>Player</Target>
</Prereq>
<Prereq>
<Type>Tech</Type>
<Attribute>Mounted_Warfare</Attribute>
</Prereq>
<AIData AIPersonality="AI_General">
<AIPrefType>AIPrefType_MOUNT</AIPrefType>
</AIData>
<ArtDef>BasicHorseMount_ArtDef</ArtDef>
</GameItemType>
<GameItemType InternalName="BasicHorseMount2">
<DisplayName>Horse</DisplayName>
<Description>Horses grant your units more speed in combat and in travel and make them immune to being knocked prone.</Description>
<Type>Mount</Type>
<CanBeEquipped>1</CanBeEquipped>
<AdditionalTrainingTurns>20</AdditionalTrainingTurns>
<HideInHiergamenon>1</HideInHiergamenon>
<ShopValue>120</ShopValue>
<ProductionRequirement>
<Type>Resource</Type>
<Attribute>Horses</Attribute>
<Value>1</Value>
</ProductionRequirement>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Attack_Boost</StrVal>
<Value>2</Value>
<Provides>+2 Attack for the first turn of combat</Provides>
<xActions>1</xActions>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_CombatSpeed</StrVal>
<Value>2</Value>
<Provides>+2 Initiative for the first turn of combat</Provides>
<xActions>1</xActions>
</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_Moves</StrVal>
<Value>2</Value>
</GameModifier>
<CutSceneDataPack>HorseMountCutScenePack</CutSceneDataPack>
<IsAvailableForSovereignCustomization>0</IsAvailableForSovereignCustomization>
<Prereq>
<Type>RestrictedAbilityBonusOption</Type>
<Attribute>DLC04_UndeadFaction</Attribute>
<Target>Player</Target>
</Prereq>
<Prereq>
<Type>RestrictedAbilityBonusOption</Type>
<Attribute>LegacyOfSerrane</Attribute>
<Target>Player</Target>
</Prereq>
<Prereq>
<Type>Tech</Type>
<Attribute>Mounted_Warfare</Attribute>
</Prereq>
<AIData AIPersonality="AI_General">
<AIPrefType>AIPrefType_MOUNT</AIPrefType>
</AIData>
<ArtDef>BasicHorseMount2_ArtDef</ArtDef>
</GameItemType>
I bolded the change. Whenever you overwrite a game item type it deletes the old item (except the artdef data), so one needs to include this on their code to fix it. Obviously you'd have to do this for the other mounts. You can put this in your mods folder and you will have the horses back in the game for the normal races.
