Here's why:
* Donkeys and mules are already hybrids: A mule is the offspring of a male donkey (jack) and a female horse (mare). A hinny is the offspring of a female donkey (jenny) and a male horse (stallion).
* Hybrids typically can't reproduce: Mules and hinnies are infertile. This is because they have an odd number of chromosomes, making it impossible for them to produce viable offspring.
Therefore, there's no animal that exists "between" a donkey and a mule because a mule is already the result of a specific combination of donkey and horse genetics.