It does not appear naturally on the map, you get it from an egg that is created from your inventory I leave you a code to put it in your game.ini that is for you to get it a little earlier because it is very complicated to get it in the game because it asks you for things that you get at the end of the game and why have a dino if you get it at the end of the game right?
I don't want to say that I make it easier, I just level it so you get it a little earlier but it has the same difficulty.
If you have any questions I will be happy to help you.

ConfigOverrideItemCraftingCosts=(ItemClassString="PrimalitemConsumable_Egg_IndominusRex_Fertilized_C",BaseCraftingResourceRequirements=((ResourceItemTypeString="PrimalItemResource_ApexDrop_Argentavis_C",BaseResourceRequirement=10.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_Rex_C",BaseResourceRequirement=5.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_Giga_C",BaseResourceRequirement=1.0,bCraftingRequireExactResourceType=false)))
ConfigOverrideItemCraftingCosts=(ItemClassString="PrimalitemConsumable_Egg_IndominusRex_Fertilized_AB_C",BaseCraftingResourceRequirements=((ResourceItemTypeString="PrimalItemResource_ApexDrop_Basilisk_C",BaseResourceRequirement=5.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_XenomorphPheromoneGland_C",BaseResourceRequirement=2.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_RockDrake_C",BaseResourceRequirement=2.0,bCraftingRequireExactResourceType=false)))
ConfigOverrideItemCraftingCosts=(ItemClassString="PrimalitemConsumable_Egg_IndominusRex_Fertilized_SE_C",BaseCraftingResourceRequirements=((ResourceItemTypeString="PrimalItemResource_KeratinSpike_C",BaseResourceRequirement=5.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_FireWyvern_C",BaseResourceRequirement=1.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_LightningWyvern_C",BaseResourceRequirement=1.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_PoisonWyvern_C",BaseResourceRequirement=1.0,bCraftingRequireExactResourceType=false)))
ConfigOverrideItemCraftingCosts=(ItemClassString="PrimalitemConsumable_Egg_IndominusRex_Fertilized_EXT_C",BaseCraftingResourceRequirements=((ResourceItemTypeString="PrimalItemResource_ApexDrop_GasBag_C",BaseResourceRequirement=5.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_Giga_C",BaseResourceRequirement=3.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_RareDrop_CorruptHeart_C",BaseResourceRequirement=50.0,bCraftingRequireExactResourceType=false)))
ConfigOverrideItemCraftingCosts=(ItemClassString="PrimalitemConsumable_Egg_IndominusRex_Fertilized_GEN_C",BaseCraftingResourceRequirements=((ResourceItemTypeString="PrimalItemResource_Electronics_C",BaseResourceRequirement=1500.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_Oil_C",BaseResourceRequirement=1000.0,bCraftingRequireExactResourceType=false),(ResourceItemTypeString="PrimalItemResource_ApexDrop_Giga",BaseResourceRequirement=1.0,bCraftingRequireExactResourceType=false)))

copy and paste in game.ini