The best picks not in lane to counter Trundle include Zeri, Jinx, and Caitlyn. These champs are all good at countering Trundle with decent win rates over 56%. Even if these Trundle counter picks are not picked in the same position as you, it's not smart to take him into these matchups. They will very likely win against Trundle.
Yet, if the other team picks either Rumble or Galio, Trundle mains should strongly consider taking him, as he easily destroys these champions with win rates over 54% against each ofthem.