Animal Crossing: New Leaf launched for the 3DS all the way back in 2012. The next release for the series was Animal Crossing: Happy Home Designer. You'd think that the team would take a bit of a break after wrapping New Leaf, and then move onto Happy Home Designer, but that wasn't the case at all. In an interview with the Washington Post, we learn that work on Animal Crossing: New Horizons started shortly after New Leaf launched.
Apparently soon after New Leaf launched, the team got right to work on what would eventually be New Horizons. That schedule of work is pretty insane when you think about it. Not only did Happy Home Designer come out in 2015, but Animal Crossing: New Leaf 'Welcome amiibo' Edition came out in 2016. While it may have seemed like a long wait for fans to get the next true installment in Animal Crossing games, the dev team has been toiling away since 2012!