A Chimera is a creature made up of two different animals that are fused together in a process known as polymerization. This spell can combine animals to create an entirely new animal which contains the best traits of both parent species (bat radar, dolphin intelligence, sharks ferocity, etc). There are many applications for a chimera. Instead of spending thousands of years breeding animals for desired traits, a summoner can simply combine the animals with the traits that they desire. They can also be used for their hides or meat, which have many uses. However, these creatures are sterile once combined, and must be custom built.
The more I think about it, the more I see a problem with this, as the spell can lead to a number of bizarre creatures. A sharktopus (shark and octopus), or piranha-conda ( piranha and anaconda), a pigagle ( pig and eagle) and other random combinations can occur. Although hilarious, this cheapens the seriousness of the magic, making the whole thing seem like a bad SYFY movie. It would also force people to honor half remembered committments they promised they would make "when pigs fly".
What mechanism can I introduce to limit the madness that would result from combining completely random creatures like this?