Kog'Maw Runes explained
There's one set of runes for Kog'Maw on patch
11.8
that is the clear favorite for LoL players and pros alike. The best Kog'Maw Runes on the position 
Adc are Precision as the primary path and
Domination as the secondary path with a pick rate of 59%. Besides runes you should take a look on Kog'Maw Build and if you are interested in pro matches take a look at Kog'Maw Probuilds. If you want to discuss
Kog'Maw then you should visit the champion subreddit.
Primary path on Kog'Maw
Precision improves attack and sustained damage on Kog'Maw troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Lethal Tempo is the best keystone rune to run on
Kog'Maw. This keystone rune gives Kog'Maw a great head start in team fights and allows to put out more damage in that time window. This is usually followed up by taking
Presence of Mind,
Legend: Bloodline and
Coup de Grace.
Presence of Mind helps Kog'Maw with the mana problems by restoring missing mana, if a champion is killed.
Legend: Bloodline gives life steal, which aids in winning damage trades and fights in the later stages of the match. Kog'Maw deals with
Coup de Grace more damage to targets with low health.
Secondary path on Kog'Maw
When it comes to your secondary path, most
Kog'Maw players prefer to go with
Domination.
Domination improves burst damage and assassination tools on Kog'Maw. On the
Domination path, the best runes for KogMaw are
Taste of Blood and
Ravenous Hunter.
Taste of Blood heals Kog'Maw as soon as you damage an opposing champion.
Ravenous Hunter is the final rune of the
Domination path. This rune gives some survivability during short fights and damage trades during the laning phase. The additional health is nothing to scoff at and can mean the difference between winning or losing a battle.