How often will a horse egg contain a donkey?

I recently heard that you can spawn a donkey from a horse egg with a random probability so I am curious: how often does this happen?

Solution 1:

According to the Minecraft Wiki:

Horse will spawn 90% of the time; donkey 10% of the time. A foal will be spawned 20% of the time.

The foal will mature into a horse 90% of the time and a donkey 10% of the time.