Simple:
* It seemed improbable that they would win the lottery.
* The theory was improbable, but not impossible.
* It's improbable that you'll see a unicorn in your backyard.
More complex:
* Despite the improbable odds, the team pulled off a stunning victory.
* The evidence presented was so improbable that the jury had to question its authenticity.
* Her recovery from such a severe injury was improbable, but she defied all expectations.
Figurative:
* He was an improbable hero, a shy accountant who saved the day.
* The plot of the movie was so improbable that it bordered on ridiculous.
* Their love story was improbable, but undeniably real.
With emphasis on unlikely:
* The idea of a flying car is still improbable, but not entirely impossible.
* It's improbable, but not impossible, that we could find a cure for cancer in our lifetime.
* The probability of that happening is highly improbable.
Remember to choose the best sentence structure and context for your particular needs.