Mob stats
Mob Stats · hud element · Krylix
The panel in the top-left corner counts the hostile mobs you have killed in this world. One row per mob type: a face and a number, most-killed first. It has no title and no background, and nothing is drawn until your first kill.

What counts
Only hostile mobs, the ones the game itself files as monsters. Animals, villagers and golems are not counted.
The kill has to be yours by the same rules as in the kill feed: your last blow, your tamed wolf's, or a mob you knocked off a cliff.
Every player has their own numbers. The server keeps them in the world save, sends you the whole list when you join and one update after each kill. They survive restarts and move with the world folder. A server without Krylix has nothing to count with, so there the panel stays empty.
Rows and the key
The panel shows 8 rows. mobStatsMaxEntries in the config takes anything from 1 to 50. Mob types that don't fit are still counted, they just aren't drawn.
Press J to hide the panel. It also hides with F1 and under any open screen except chat. There is no option to move it.
The Mob Kills tab of the leaderboard shows the same kills as one total per player.
Worlds from 1.3
Before 1.4.0 the panel showed one count for the whole world, shared by everyone in it. Nobody recorded who killed what, so 1.4.0 can't split it. In a world where exactly one player has statistics, which is what a singleplayer world looks like, the old counts become that player's. In a multiplayer world the old pool is left alone and every panel starts empty.
The Mob Kills totals on the leaderboard were per player before 1.4.0 as well, and they stay. In such a world your leaderboard number can be higher than your panel adds up to.