As mankind began moving around the world he introduced animals to new areas, many of which are now destructive pests ... https://en.wikipedia.org/wiki/Feral
There are unexpected effects from ferals. An example, the black rat ... from Wikipedia:
"After Rattus rattus was introduced into the northern islands of New Zealand, they fed on seedlings, adversely affecting the ecology of the islands. Even after eradication of R. rattus, the negative effects may take decades to reverse. When consuming these seabirds and seabird eggs, these rats reduce the pH of the soil. This harms plant species by reducing nutrient availability in soil, thus decreasing the probability of seed germination."
Some ferals are very successful. For example, the brown rat:
"The only brown rat-free areas in the world are the continent of Antarctica, the Arctic, some isolated islands, the Canadian province of Alberta,[66] and certain conservation areas in New Zealand."
Some ferals were introduced to solve problems, but became far worse than the original problem. For example, the toxic cane toad has infested the northern half of Australia ... https://en.wikipedia.org/wiki/Cane_toad