I am sorry I cannot identify this plant from the description, the chance are that it is a weed as you have not planted it. As the seeds of the plant are large the seed it grew from could have been carried into your garden by a bird. Some plants can grow from seeds in bird droppings.
I think it may have had such small, insignificant flowers at an earlier stage of growth that they were not noticed. I cannot think of any plant that produces seed without some form of flower.
Someone else may be able to identify it better than this, as I live in the UK and our climate means we have different wild flowers and weeds here.
If it is a weed try to make sure it does not set seed in your garden, if you want to see what the plant seed pod develops like leave only one on the plant. You could try a Google images search with a description of it but you would be lucky to find the right plant image. You could take a photo of it and ask what it is on a gardening web-site someone might recognise it. Hope this helps.