Cotoneaster is more of a shrub than a tree, but nevertheless they are superb to use as Bonsai. They are extremely versatile and can be shaped very easily into whichever style you wish. They are one of the species that work particularly well with the "clip and grow" method. Clip and grow is a pruning/shaping technique meaning you often don't need to wire each of the tiny little branches, you can simply shape the pads and leafed areas by clipping off the undesirable bits and letting the tree grow in the areas you want to fill out.
An advantage of this technique is that the tree can often look more natural. Because the branches are constantly being regrown they maintain the correct proportion for the size of the tree.
This is one of our own trees, aged around 10 years, and shaped into a lovely informal upright with tight corners and lovely thick pads. The tree is a fruiting variety so in Summer it is full of small green berries. Later they will turn a vivid red and the tree will take on a different look altogether. The leaves are a fairly dark, very vivid green, which looks incredibly against the bright red berries.
We bought this tree from a local Bonsai expert, John Hanby, and it is one that we won't be selling, but will keep for our own private collection.