I grew up in the black and white television days of monsters like the Mummy and Frankenstein. The true vampire Dracula and the one I am going to nominate for; what monster from that time period could exist today the Werewolf.
The reasons I picked the Werewolf are the following, given the large homeless population in this and every country around the world he could live among and prey on them. Stay with me hear for a minute or so, he only kills on a full moon. Then he prey's on those that for the rest of the us have disappeared into the world of oblivion. Plus it could be blame on some that is mentally ill in that same group of those that have disappeared into oblivion.
The Werewolf could also exist in a rural setting think about it, he could grow a cash crop like corn or wheat plus soybeans or hay to feed the livestock that he would need. If every once and awhile some of his livestock were killed and he lived in an area were wild animals could be blamed why not.
Finally the large natural forests and mountain ranges plus unexplored lands that we have in this country in Alaska plus Canada last but not least South America.
My least likely would have to be the Mummy who would take something lumbering along wrapped head to toe in bandages seriously.