Paul warned the Ephesian elders that after he left, savage what would come in among the flock?

Answer: Wolves

Paul told the elders to shepherd God's church, warning that savage wolves (KJV: grievous wolves) would not spare the flock.

“Keep watch over yourselves and the entire flock of which the Holy Spirit has made you overseers. Be shepherds of the church of God, which He purchased with His own blood. I know that after my departure, savage wolves will come in among you and will not spare the flock.”— Acts 20:28-29 (BSB)