Dragon slows attack speed.

Guaranteed reproduce:

Pick Tryndamere.

Farm the jungle, get a stalkers blade.

Go fight dragon at level 5 or 6. The first time the dragon auto attacks you the attack speed of your champion will be slowed.

I'm not sure if these steps can be modified to produce the same results; but it happens to me reliably in ranked games. Its not something that might or might not happen, it happens 100% of the time. Only the first dragon slows attack speed, and only on the first attack, but it lasts for about 4-5 seconds. The other dragons don't slow attack speed.