@Gershon's answer is spot on. But just in case not everyone agrees, I will offer another. You state in your comment that there are many ma'amarei chazal which state that Avraham was the first person to circumcise himself. This leads you to the presumption that Terach did not have a bris at all.
However see Rashi on 15:15 from Bereshis Rabbah 30:4 that Terach did Teshuva toward the end of his life. In that case it is very probable that he also had a bris done afterwards. His death was long after Avrahams own Milah. (205[Terach died] - 70[Terach at Avraham birth] = 135).
It has just occurred to me that I am not sure why Terach was in Hell seeing as he had done Teshuva but I suspect that is a contradiction not based on my answer. Perhaps I'l ask it separately.