Question: when was the last wolf pack killed in yellowstone?
Answer: by 1926, 1926

When Yellowstone National Park was created in 1872, gray wolf (Canis lupus) populations were already in decline in Montana, Wyoming and Idaho. The creation of the national park did not provide protection for wolves or other predators, and government predator control programs in the first decades of the 1900s essentially helped eliminate the gray wolf from Yellowstone. The last wolves were killed in Yellowstone in 1926.  After that time, sporadic reports of wolves still occurred, but scientists confirmed that sustainable wolf populations had been extirpated and were absent from Yellowstone during the mid-1900s.