Strange looking pumpkin

One of my pumpkin plants has produced a pumpkin that looks different from all the other plants. I planted some Sankt Martin and some Crown Prince, and they all look the same except for one Sankt Martin plant which has produced this. It's the only fruit on the plant. It looks more like a courgette except it's much fatter than my courgettes. Any idea why this is?

It's probably a rogue seed that's got into the packet. One of my Baby Bear pumpkins is obviously not any kind of pumpkin it's growing as a Bush not trailing and the fruit on it is more like a fat courgette. I'll leave it on just to see what it grows in to