How do you use a Black Stallion?

[Mete Han]

I've seen this mentioned elsewhere but could not find a concise answer.

Several of my assassins now have Black Stallions, which according to the description on the packet will increase the speed of my army by 25%.

However, I can't seem to persuade an assassin to give his horse to a general, and putting the assassin in the army stack doesn't seem to make any difference to its movement.

Can someone just explain what I need to do?

You can't be serious