Amumu is still a yordle, and I can prove it.

In the documentary about the Curse Of The Sad Mummy music video we get a brief (and I do mean brief) glimpse of his doggy ears.

http://tinypic.com/r/35kk02h/9

http://tinypic.com/r/ippq8w/9

http://tinypic.com/r/1pw392/9

If you don't believe me then look for yourself. You may need to pause and play the video a few times before you see it.

https://youtu.be/4bQBmAO0hk4?t=2115

35:15 to 35:17