A ]Kog that that can tank? its true! By going for the Heart of Gold first for gold, then getting Warmog's Armor, you will easily have about 3.5k health by the time you hit level 18. And that is just the start!
Now, let me preface this with THIS IS NOT FOR EVERYONE. Most people would either like to be a MELEE tank or a ranged DPS, not a RANGED TANK. However, it tends to catch people off guard for one, which is a huge advantage. And while you have no CC abilities, by level 12-13, you can take a huge amount of punishment and survive, meanwhile dealing a large sum of damage to the enemy. It is even possible to SOLO KILL a TURRET, without any minions. (Yes, I have done it, though admittedly, i had to "b" immediately afterwords...)
Then add the bonus mana (i don't know about you, but i tend to run thirsty...) and Magic resit, you can hold enemy champions well, even tank kill turrets and more. The runes help boost your AS, while most of the items add health, mana, and/or armor, a few all three. While it takes a bit to get it up, and a good partner (if you are not soloing), once you have the Warmog's (easily by lvl 8-9 with the right people), you can even float and help where ever it is needed.

No big mystery, all of the runes are for Attack Speed. Obviously, there are two options there, the Flat runes, which boost your early game better, or the Scaling runes, that boost your late game better. As Kog tends to have most trouble early on, Flat is better, hence the Alacrity runes.

You could alter them to add armor, mana, or health, and go full tank mode, but as Kog tends to have a bit of a slow attack speed, a little boost is a good thing. Also, as your basic attacks to a fairly large amount of damage with Bio-Arcane Barrage activated (which, with the cooldown only slightly longer than the duration of the spell (about 4 seconds more depending on ability level), it should almost always be...), you can turn yourself into a nuke tank by boosting your attack speed through your runes.

While a tank would normally focus on defense, as you are a DPS based character, the Attack line is the best line to focus on at first, as you can produce a TON of damage up front, and boost the natural capabilities of Kog'Maw. Once you have a fair amount in the attack line, a little bit of defense is key, to help with the tanking side of life.

The items are focused on 2 things mostly: survivability, and getting more items faster.

Survivability is covered from the start with the Ruby straight off the bat. Then, once you get a bit more gold, you can get the Heart of Gold, which will make all future purchases easier, with its unique passive (+5 gold every 10 seconds). From there, the most invaluable item to get early is the Warmog's Armor, with its passive that grants you PERMANENT boosts to your health with each kill (yes, minions as well!) Just that alone can get you to 3.5-4k hp by level 18. Boost that further with armors (namely Sunfire Cape and Randuin's Omen), and you can hit 5k+ health! The last item, Force of Nature, helps with the magic resist, while still providing additional health regen, and a boost to movement speed (one thing that Kog lacks with this build...)

The one item that does not focus so much on survivability addresses the one major issue with Kog: Mana. While you can spam the ult, and do a ton of damage with it, it is a very mana expensive ability to spam, and tends to run Kog a bit thirsty. The Archangel's Staff boosts not only your max mana both flatly (+400 mana), but passively as well, granting an additional +4 mana for each ability used, to a max of 1k bonus, and it also gives you a mana regen, 25 every 5 seconds. That is not the best part though. It boosts your ability power by 45, PLUS 3% of your max mana.

Obviously, this build is more for going against melee/physical damage opponents. If you are going against more casters, the best option is to swap out some of the high armor items and get magic resist instead.

Also, I did not put any pots in here, as I assume if you know how to use them, you also know when you need to get more. So yes, get pots as you need them.

Unlike most Kog builds, especially early on, you might find yourself running away from a gank. Having Void Ooze helps by not only damaging the enemies, but also slowing them down.

Without a doubt however, the most potent ability, other than your ult, is Bio-Arcane Barrage. Not only does it increase your range (by a rather large amount in the upper levels (130/160/190/220/250), but it deals additional damage per attack (who does not like that???).

Caustic Spittle, however, while you get the passive AS increase, does not up your damage and killing/surviving ability overall as the other two.

And the ult, Living Artillery, well, obviously, you want that maxed out ASAP. Not only does it get a HUGE range (farther than you can see even), it does MASSIVE damage if you hit, and reveals the area for a second, and if you score a hit, you gain visibility on the target as well (which works well against people hiding in bushes).

Using your two free-targeted abilities (Void Ooze and Living Artillery) is a great way to make sure that patches of bushes are clear, and avoid enemy ganks (and trust me, they will gank you as soon as you get Warmog's and start stacking it's buff)

While on Skills, while it is your passive, DO NOT FORGET ABOUT Ithican Surprise!!! It can mean the difference between a death, and a 1 for 1 trade, or more! And you would be surprised how many people do not know to run from it! I have taken out 3 of the 5 enemies with it! It is very powerful, especially late game against squishies, where at level 18 it deals 550 TRUE damage to near enemies 4 seconds after your death. Revenge can be very sweet, not to mention instant, indeed!

The summoner spells really can be any movement spells, but as the items do not show boots until the third, mobility is key, and Teleport is the king of mobility spells, despite it's 5 minute cooldown (3 minutes if the the spell fails for any reason). Obviously the best use of it is to port right into the fight after a Recall ("b") or, god forbid, death.

As Kog can be thirsty, especially after gaining Living Artillery, Clarity can boost your push even longer by restoring not only your mana, but that of other friendly champions near by.

Other viable options are Ghost and/or Flash, which can get you out of tricky situations faster, thus raising your survivability, or Heal, for obvious reasons, or Exhaust, which is more offensive, but also can get you out quickly.

This build is very heavily reliant on a good teammate in your lane (or good holders on the other two if you go mid). In my experience, either a good melee off-tank (eg, Wukong) or a powerful spellcaster/assist (eg, Morgana) can get you off to a great start. Once you get your Warmog's and Living Artillery, however, you can start to solo a dual-lane (top or bottom), or float and squash major enemy pushes. At that point, you become a major push factor for your team, and can even backdoor unattended lanes with minimal difficulty.

PROS:
-Highly increased survivability
-Unexpected
-can solo 1v1 or even 1v2, and even turrets

CONS:
-very team based
-heavily carry early on

While you would not necessarily think it based off of what you see here, you will still dish out massive damage. 1 hit from Living Artillery can cut most squishies down by 25% or more when fully maxed out.

Also, obviously this build is designed for classic game mode, as dominion does not have teleport available. However, with a few minor adjustments, it can be converted. 1) Swap our Teleport for either Ghost, Revive, or Flash. 2) Get a pair of boots early, either the Ninja Tabi for armor and dodge, or Mercury Treads, for the Magic Resist and Tenacity.

In all, I know that this is not going to be for most Kog players, as it is well off the beaten path. But, I find it works well, for my style of play. Even if you don't play this way, please leave a comment, and let me know what you think. This is a very team-oriented build, and requires a large amount of teamwork. Once you try it, let me know what you think, and be candid, but constructive please! Things like "this is stupid" "this doesn't work" etc doesn't help. tell me WHY. that way, I can make it even better for you all.

Anyways, have fun out in the fields, and if I meet you there, I wish you GOOD LUCK!