In addition to the Kholodok seeds Darrell sent, I’m growing King Winter and Wintermelon watermelons along with 5 or 6 other watermelon varieties. I’m not in for saving seed at present, so cross pollenation is likely gonna occur in my garden.
I just saw this one and had to respond even if 2 years old. I would put my money on the culprit being a rabbit. Look for rabbit poop nearby if it happens again. They always tend to leave their calling card! 