You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
alternatehistory.com
Ever since the last ice age there are many empty ecological niches in North America. The whole continent really only has a handful of large animal species where environmentally there should be dozens. For example wild horses have done exceptionally well as they fill the gap left by extinct ancient horses. Over the last two centuries there have been several proposals to introduce large animals to fill those niches.
Besides horses and cattle what other large animals can you reasonably see being introduced as North America is colonized? Nobody is going to want lions running around the midwest. I personally like the idea of seals being introduced to the Great Lakes for their fur.