FOR FOALS:
First, press CTRL + F and search for "Corral". You're going to want to find every instance of said word and paste the following in with the other entries for it:

        <Item>
          <Name>A_C_HORSE_FOAL</Name>
          <Variations type="CAmbientPedModelVariations">
            <LoadOut />
            <Outfit />
            <Health />
            <DomesticatedLevel>ADL_Domesticated</DomesticatedLevel>
            <RarityLevel>ARL_Common</RarityLevel>
          </Variations>
          <Probability value="1.000000" />
          <TimeOfDay />
          <InventoryInfoHash />
        </Item>


Then, find ANIMAL_HORSE_WILD_AMERICAN_STANDARDBRED and paste the following in with the entries for that:

        <Item>
          <Name>A_C_FOAL</Name>
          <Variations type="CAmbientPedModelVariations">
            <LoadOut />
            <Outfit />
            <Health />
            <DomesticatedLevel>ADL_Wild</DomesticatedLevel>
            <RarityLevel>ARL_Common</RarityLevel>
          </Variations>
          <Probability value="1.000000" />
          <TimeOfDay />
          <InventoryInfoHash />
        </Item>


FOR WILD HORSES:
Search for "Horse_Wild" and delete all other horse entries except one and the entry for the Criollo, then replace that one horse with A_C_ANDALUSIAN_PERLINO