Chimeras are formed from four parent cells (two fertilized eggs or early embryos fused together) or from three parent cells (a fertilized egg is fused with an unfertilized egg or a fertilized egg is fused with an extra sperm). Each population of cells keeps its own character and the resulting animal is a mixture of tissues. Inevitably it would be quite difficult for a trans-species chimera to occur let alone develop functionally. In order for a Chimera to be created some form of unusual gamete fusion has to occur, incorporating more than a single gamete from each parent (as addressed above). It would be near impossible for this to occur between different species as different species' gametes cannot fuse (except in the case of closely related species as in mules). The more distantly related the organisms being "fused" the less likely the fusion.
As the organism develops, the resulting chimera can come to possess organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosomes and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, and at one time was thought to be extremely rare, though more recent evidence suggests that it is not as rare as previously believed. Most will go through life without realizing they are chimeras. The difference in phenotypes may be subtle (e.g., having a hitchhiker's thumb and a straight thumb, eyes of slightly different colors, differential hair growth on opposite sides of the body, etc) or completely undetectable.
Sometimes the Chimeras have different colored eyes (heterochromia) or may have altered sexual functioning (such as being a hermaphrodite), and in that respect they can be "creepy", however all of the addressed chimeras are still two different lines of heredity from a single species.