Bull

An optimist; someone who assumes that prices will rise. The origin is unknown, although it probably evolved because it contrasts strongly with bear. As the quote from Alexander Pope shows, it was in common usage in London by the early 18th century.

Come fill the South Sea goblet full;
The Gods shall of our stock take care:
Europa pleased accepts the bull,
And Jove with joy puts off the bear.
Alexander Pope, inscription on a punch bowl, 1720
(the year of the South Sea Bubble)