Of course, there are some legendary animals from ancient bestiaries that are quite impractical. Take for instance, a dragon. It breathes fire which makes it ineligible for the average apartment. It reeks of sulfur. It's food is restricted to virgins (a rare commodity) and the size of the pooper scooper would be somewhat unwieldy. Plus, you'd have to let the thing fly for exercise and, as most dragons are the size of a Piper Cub, they would endanger an already congested airspace. Plus, and forgive me for being vulgar, a dyspeptic dragon flying over my house is not something I find pleasure in meditating upon. No. If one wants a dragon that's convenient, best to settle for an iguana.
Mermaids would also be a tad tough. Salt water aquariums are expensive to maintain and you'd need a large one for a mermaid. Plus, when they die, they don't conveniently flush down the toilet.
Dryads would be cute, that is if you get off on anthropomorphic tree spirits. You'd need quite a large lawn and spraying for pests would be important. However, I don't know if I could trim a tree that screams.
No, of all the beasts of legend that an enterprising biologist could attempt, I believe the unicorn was made for Madison Avenue.
Consider. First of all, a unicorn is a horse-like creature with a lion's tail and one horn coming out of its forehead. Well, we already have access to DNA for horses and lions. The horn would be a challenge, because there is a seam that runs down the front of an animal's cranium. Better if you had two thin horns grow out of each side of the seam and intertwine as they grow. You could still have a two-horned creature with the illusion of one horn. No big deal. Even the purists could accept that.
However, speaking of purists, the genetic construction of a unicorn is not without its challenges. Historically, unicorns are always referred to in the masculine. No problem there. With no female unicorns, breeding becomes impossible and each masculine unicorn must be individually created in the creator's vat of genetic soup. That would supply the business with a nice, steady cash flow.
The horn of a unicorn has the reputation for purifying water. It would be a formidable task to the average geneticist, but if the horn could exude a natural chlorine compound, it could do the purifying trick quite nicely.
It is when the purists insist on a historically correct unicorn that only hangs around virgins that we find ourselves on the proverbial horn of a dilemma. How the heck would a biologist handle that one?
We know that humans give off pheromones. If the pheromones of women change through certain life experiences, you simply engineer a unicorn that is repelled by certain human female pheromones. They could even engineer the unicorn to become sleepy at the aroma of virgin female human pheromones so they would want to doze off with their heads in their mistress' laps!
And for human males? In legend, unicorns don't hang around with men anyway. As psychiatrists are discovering that most psychoses and neuroses are mostly through brain chemistry, it is a simple matter to engineer the unicorn as totally hostile to the presence of males.
So far, I've described a beast that would make a great watch dog for the average teenage daughter ... a living chastity belt that can kill.
However, sleepy, man-hating, horned unicorns may not be what Madison Avenue wants. Why restrict the ownership of unicorns to a very small portion of the human population?
And who would want a pony-sized beast cantering around the average home knocking over bric-a-brac, begging at the table (when it's not trying to kill your son), having to take it for walks in areas where males were not present, etc.?
Why not make the unicorn intelligent, anthropomorphic and available in both genders? In fact, make it a homophile as well as sycophantic and you have a perfect, if not a tad clingy, companion for most single men and women. And if the creature can cook and do light household duty, you won't be able to keep them on the shelves.
Just don't push it to the point where it experiences rejection. You might end up with a unique stab wound!