The prediction market Polymarket is holding early conversations with investors and looking to raise money at a valuation between $12 billion and $15 billion, more than 10 times its value just four months ago, according to people familiar with the matter.Polymarket’s valuation has ascended quickly. In June, Peter Thiel’s Founders Fund led a $200 million funding round for the startup, valuing it at $1 billion. Earlier this month, Intercontinental Exchange Inc., owner of the New York Stock Exchange...