New Delhi, June 16 -- A single bettor on the prediction market platform Polymarket lost close to $1 million when Cabo Verde held European champions Spain to a draw on Monday. What oddsmakers viewed as a near-certain Spanish victory turned into one of the most unlikely results in the opening days of the World Cup.

Spain arrived as one of the strongest teams in the competition. A Goldman Sachs model gave them a 26% chance of winning the entire tournament. Cabo Verde, appearing in the World Cup for the first time and without any high-profile professional players, were heavy underdogs. The draw added to a string of surprising results, including Japan's late equalizer against the Netherlands the previous day.

The standout figure for Cabo Ver...