𝗧𝗵𝗲𝗼𝗿𝘆 𝟭: Horses arrived in the Cordillera region way before the Philippines was colonized by Spain. Chinese historical accounts show that there were busy trading ports in what is now modern Pangasinan and La Union. Merchants from China and Japan arrived at these ports to do business. Merchants during these times didn't just trade goods, they also traded animals. It's possible that their ships came with horses.
It's also an established fact that pre-colonial Igorots often went down to the lowlands (Agoo, etc.) to trade. So it's possible that Igorots bought horses with their precious gold.
However, this theory is contested by many historians citing that there's no record of horses from China/Japan being traded in Pangasinan/La Union trading ports. They add however that there's evidence of donkeys being carried by these trading ships. A Chinese shipwreck for example, contained donkey bones, not horse bones.
𝗧𝗵𝗲𝗼𝗿𝘆 𝟮: Malaysians brought horses to southern Mindanao (also in pre-colonial times). Through trade and ports, these horses made their way to Luzon and eventually to the mountains of the Cordillera region. There's very good evidence that horses were indeed brought to Mindanao from neighboring Malaysia. However, there's no evidence that these horses found their way all the way to Luzon.
𝗧𝗵𝗲𝗼𝗿𝘆 𝟯: Horses were introduced by the Spaniards. This is the most accepted theory as it's backed by historical evidence and accounts. The Spaniards tried numerous times to put the Igorots under their control. Expensive expeditions were launched to map the Cordillera region and subjugate the local populations. Hundreds of horses were used during these expeditions.
The Spaniards were able to set up many command posts (comandancias) within the Cordillera region. Travel to these comandancias often utilized horses.
*The accompanying image is from the digital archives of the Peabody Museum of Archaeology and Ethnology. It shows a young man and a pony in Bontoc, Mountain Province. Taken between 1898 and 1912.
