Origin of Turian's Creatures

Turian possesses many different species of animal- and plant life.
In fact, there are differences in the general set-up of the animal groups, and to a lesser extent, the plant life, that many scholars never ever had a doubt about the truth in the legends or any myths surrounding the creation of Turian's World. A quadrupedal body plan is as common, or uncommon, as a hexapedal one, with the occasional quintapedal and other body forms to be found as well. The number of "primitive" variations of lifeforms that are around when obviously much more advanced versions are around as well, some even sharing the same environment and prey is considered the most significant indicator that Turian is a created world. Also, the huge number of purely artificial species gives credit to the creation myth.
Various sentient species with their histories, many of which did not originate in Turian but have been imported or migrated are further indicators.

"My dear colleges, how could we even entertain a thought of something so ridiculous as an evolution of life, when evidence of a creative mastermind is so obvious? Not even the evolutionists deny that evidence and thusly I really think we can agree on the origins of our world."
Magistra Weilenfus of the Imperial Academy commenting for the 7th edition of Myths and Facts - A Study of Creation.

Species originating from a different evolutionary paths are generally not compatible with each other, they can not produce offspring. Meaning there are, for example, no human Lizardmen cross breedings, also species from different realms, universes etc. are most often not compatible even if they are "in world" considered to be identical, they are not.
That does not mean there couldn't be a half-elf or half-orick etc. just that they are not a natural occurrence. They are either their own species and the name is just a simple depreciatory convenience or they are the result of powerful magic, genetic engineering etc.

The so-called half-orks are a very good example of this. Their appearance may look like a combination of orick features with human ones but there is not a single known specimen that can trace to either an orick or a human ancestor, instead their folklore indicates that they are an engineered species.