There are always new styles being devised by bonsai artists. Some of these styles are very unique, and probably came about due to the chance acquisition of an unique bonsai yamadori that happened to bear some resemblance to some object. So, the bonsai artist merely built upon such a base foundation, to accentuate the tree’s unique features, and heighten that resemblance.
On the other hand, most other unusual forms of bonsai styling are products of creativity, and patience. There is no limit to creativity in bonsai, except that it takes patience to actually make those ideas work.
One this note, two unusual styles in bonsai are the Raft style and the Group style.
The Raft Style is styling the trunk to grow at an angle to the ground, sometimes parallel to it. The trunk may be touching the ground, and is meant to portray a tree about to topple, but otherwise still surviving, standing, and even thriving.
Sub-styles include the ikadabuki (straight line), in which the trunk is entirely out of the soil. Typically it will rest on the surface, but some can actually grow somewhat like a kengai and are slightly above the ground.
In another sub-style, the trunk rests at an angle, partially underground. In this case the bark under the earth will tend to decay from moisture and small soil organisms. The netsunagari (sinuous) style is one of the more exotic sub-types. Here the roots meander through the soil like many underground rivers and the trunks are highly gnarled and twisted.
Sometimes, the trunk will be allowed to sprout multiple branches that resemble individual trunks. These appear similar to a group or forest style, but all grow from a single tree, and may look like a mini forest from afar. This is also called the Kabudachi style. The grouping of trees together, or at least the visual effect of grouping them together is collectively called the Yose style.
This then brings us to the Group/Forest style, or Yose style (in Japanese).
This style consists of multiple trees growing together within a single container. It is meant to resemble a miniature forest, and for the best aesthetic effect, only one single species should be used – although you can use any species for this style. Mixing up the species would also make it harder to water and feed the trees, since every species have different individual needs.
These two styles are but two out of many other complex, and unusual bonsai styles. The choice of whether to create such exotic styles is dependent on experience, skill level, and of course – how much you would want to see your bonsai composition succeed. The blending together of competing influences, is also one of the underlying characteristics of bonsai design. The goal of competently displaying harmonious elements in an otherwise inharmonious setting, is one of the constant characteristics of any unusual bonsai style.