Answer:
Because we domesticated them
Step-by-step explanation:
Ex.1
Wolf
Ohhh... so hungry, oh those cave guys have some leftover mammoth meat. If I look cute and helpless they may give me some. So he puppy dog eyes them and whimpers and they give him the meat. Soon they decide that getting food is better than hunting for it and BOOM we have today's domesticated dogs.
Ex.2
Tegu
We decide that they are beautiful animals and ethier pick it up out of it's natural habit as a baby or we get it at a pet store where someone else did the exact same thing. But... Eventually we release that animal when it gets bigger and older and they take over the ecosystem they live in if they are a predator. The tegu eats the eggs of the others in the ecosystem because the are egg-eaters. They can completely wipe out an existing species by targeting their nest and eating their young or there eggs.
So here are two examples of this happening either they come to use or we find them.