FIrst - how much, if anything, do you know about bonsai - about how to take care of trees in little pots? Because without even the basics, you're going to have some trouble, not to mention the trouble the trees will have waiting for you to learn :). I suppose your next step here would be to cut down those 'trunks' to an inch or so above the lowest leaves and buds, to encourage new, smaller branches to grow and also creating the illusion that the tree's older in relation to the small branches, etc. But really, your first step would be to find a local club, go learn as much as you can before either digging up anything else that's alive and trying to stay that way, or spending money on stuff from stores without knowing what to do when you get them home... they won't wait for you either (sorry!). And there's a lot to learn... seriously. There's no 'instant' bonsai, certainly nothing that will last a year or more without knowledgable care... and pruning is the least of it. Every tree (of potential thousands) has different needs, in different seasons, in diff. environments, with particular soil mixes, sun and water requirements, etc. etc. so you really do need lots of patience.

And your question - 'Is this a bonsai or tree with flat roots' tells me you need to learn... bonsai are just regular trees trained over years to live in small pots, and their root systems are trained and worked on to have relatively small flat growth - but on purpose, not because they're one type of tree or another. Trees in nature may well have flattish root systems, but very often that's because they're growing in rocky soil, with little space for roots to spread out, not because they're intrinsically flat. Some trees (e.g. maples) are more likely to spread roots sideways (laterally) than others, but that's incidental to which maples and their individual growing conditions. If I were you, I'd put this one back into the ground near where you found it, in about a similar light receiving place, and go find that club.

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