Drain the horseradish well, extracting and reserving as much liquid as possible. Cut the cream cheese into chunks and place into a medium saucepan. Add the cheddar cheese. Melt the cheeses over low heat, stirring frequently to blend (or heat in microwavable bowl at 1-minute intervals, stirring in between until melted). Remove from heat and add the horseradish and mayonnaise. Combine well. Use the reserved liquid to thin the mixture if necessary. Refrigerate for flavors to meld. Serve chilled or at room temperature with crackers or raw vegetables.