All placeholders have the same syntax: %mkc_<MobName>%
All possible arguments to <MobName>:
zombies, skeletons, spiders, cave_spiders, blazes, creepers, ender_dragons, endermans, endermites, bats, chickens, cows, ghasts, guardians, horses, iron_golems, magma_cubes, mushroom_cows,
ocelots, pigs, pig_zombies, sheeps, silverfishs, slimes, snowmans, squids, villagers, witchs, withers, wolfs, wardens, allays, axolotles, bees, piglins, brute_piglins, or zombified_piglins.

For example: %mkc_zombies% or %mkc_iron_golems%.

This is an example of a scoreboard with this placeholders:

/mkc page - Use it to see the plugin page and see detailed info about the plugin.
/mkc placeholders - Use it to see the available placeholders.
/mkc version - Use it to see the plugin version.
/mkc reload - Use it to reload the config.
/mkc permissions - Use it to see the plugin permissions.
/mobkills - Use it to see all mobs you killed.
/mobkills <player> - Use it to see all mobs a specific player killed.
/mkc setreward <name of the mob> <kills for reward> - Use it to set a reward if someone reaches a specific kills of specific mob.

PS: It is recommended not to edit the players.yml.

mkc.version - /mkc version
mkc.reload - /mkc reload
mkc.mobs - /mobkills
mkc.setrewards - /mkc setreward
mkc.placeholders - /mkc placeholders
mkc.updatecheck

The config is very simple:

With the Send-Message-insteadof-Command option, you can send message without the console if the command is /msg or /tell, this is because some plugins like ChatManager blocks the sending of messages with console.
With this option you also can send the messages with colors.

IMPORTANT NOTE
If you have updated the plugin from version 1.x to 2.0, and you have some rewards configured, you will have to modify the config so that the rewards you had are adapted to the new format. All you have to do is, next to each reward number, make a line break and add a "-" and a space. And so it would be with each reward.
Example:
Before:
Rewards:
villagers:
10: tell %player% &cYou killed 10 villagers!

Later:
Rewards:
villagers:
10:
- tell %player% &cYou killed 10 villagers!

This is the format in which players who kill mobs are saved in the players.yml.
As there are very large servers I have decided that in this file only players who kill at least 1 mob will appear.
If a player enters the server he will not appear in this list but when he kills any of the mobs compatible with the plugin it will appear along with his UUID, nick, and number of mobs he has killed.