Yes, billionaires can become poor. While being a billionaire typically signifies a high level of financial success and wealth, it is not a guarantee of financial stability or security. 

Several factors can contribute to a billionaire losing their wealth, including financial mismanagement, poor investment decisions, legal troubles, and changes in market conditions.

Some examples of billionaires who have lost their wealth include former Enron CEO Jeffrey Skilling, who was convicted of fraud and insider trading in 2006 and lost much of his wealth as a result; former New York City Mayor Michael Bloomberg, who lost billions of dollars during the financial crisis of 2008; and former hedge fund manager Bernie Madoff, who was convicted of operating a massive Ponzi scheme in 2009 and lost much of his wealth as a result.

It is also worth noting that the concept of being a billionaire is relative, and the value of wealth can fluctuate over time due to various factors, including inflation, exchange rates, and market conditions. While it is possible for a billionaire to lose their wealth, it is also possible for their wealth to increase over time, depending on their financial decisions and the state of the economy.