I bought a hairbrush and I didn't know it was made of pork hair. I want to know if it Halal or Haram.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Muslim scholars are not agreed on the issue concerning the impurity of the hair of pork. Some think that the skin of the pork and all that is on it, like the hair, is impure. This is the opinion of the Hanafi, Shafi and Hambali.
However, the Malikites consider the hair of the pork pure and believe that it is lawful to profit from it. This last opinion is the most preponderant and the most correct.
Ibn Taymiyah said: 'The most preponderant opinion is that all hairs are pure: hair of the dog, the pig, and others. But their saliva is impure. So, if the hair of the dog is wet and touches the clothes of somebody he is not asked to clean those clothes because originally all things are lawful in themselves unless there is evidence proving the opposite. So the hair and feathers of any animal considered impure are pure' .
So, there is no harm in using this brush made of the hair of pig.
Allah knows best.