I killed a Quill Demon Rare in Act 1: Fields of Misery - The Scavenger's Den, level 1 just now. Inferno MP1.

No check mark has appeared.

The follow previously is the same for:
"A Rare Phenomenon"
1. Quill Demon (now confirmed)
2. Sand Dweller

"Champion Collection"
1. Fallen Cur
2. Plague Swarm
3. Subjugator

I know the check marks appeared for Uniques: Bellybloat the Scarred and Zhelobb the Venomous in the same game. So I do get credit for killing monsters.