New to Smogon's website as a member, looking at how things are right now in Pre-Poke Bank, there aren't a lot of 100+ Base Speed Pokemon, those that do have Base Speed of 100 or above have rather average to bad Defense and/or Special Defense. Post-Poke Bank things are going to look rather bad for Tapu Koko since we'll most likely see Electric-Type checks such as Lando and/or Lightning Rod users.
I managed to catch Tapu Koko in my Sun File with a Naive Nature with perfect 31 IVs in HP/Atk/Sp.Atk/Spd but looking at the stat checks, the Defense and Sp.Def IVs were less as they were 9 and 8 respectfully. So my Tapu Koko had Lvl 60 stats as the following: 172/112/89 bulk and offensive stats of 161/137/196. At Lvl 50 (the only level where Pokemon are auto-set to in online battles) with the given EVs would work possibly: +120 Atk/+136 Sp.Atk/+252 Spd. At Lvl 50 with the IVs in the following and with the given EVs: 145/94/75 bulk and 150/132/200 attacking power. This allows Tapu Koko to outspeed non-positive/neutral speed nature Pokemon whose Base Speed doesn't exceed 147 and tie with neutral speed nature Pokemon that have 148 Base Speed (I don't think any Pokemon has that Base Speed to be honest), this isn't factoring in Tailwind, Trick Room or Choice Scarves and not factoring in Speed Buff setters. Excluding Speed-buffing items or stat buffs, the given nature, IVs and EVs also allow Tapu Koko to tie with positive 130 Base Speed opponents (it will generally outspeed positive Base Speed opponents whose Base Speed is below 130). With 200 Speed, it can manage a tie with Choice Band Jolly Aerodactyl (the only non-Legendary pre-Poke Bank Pokemon that ties with Tapu Koko for positive Speed) that could easily knock it out with an Earthquake (who would go first would vary and be risky) by either risking a possible KO with Wild Charge or a Thunderbolt or U-Turn'ing out to a Pokemon that is either resistant or immune to Rock/Ground-Type attacks. Again, this set isn't meant for Tapu Koko to stay in after attacking once, due to Koko's 'average' bulk, it's meant to fire off a U-Turn and call in for a different Pokemon to handle the opponent's current Pokemon if said opposing 'Mon is meant to deal with Koko. Koko isn't a very defensive Pokemon so it isn't recommended to keep it in for a long time, especially if you run Life Orb.
Items would either be Terrain Extender if I'm wary about Sleep-users come in after Electric Surge would end (looking at RestLax and RestBro) or Life Orb for additional power although the recoil damage would eventually rack-up to be a pain.
Moves:
U-Turn: Allows Tapu Koko to switch-out to a safer Pokemon if you can potentially read the opponent's moves in that they'll switch to a Ground, Electric or Grass-Type Pokemon to counter/check Tapu Koko.
Wild Charge/Thunderbolt/Electro Ball: 150/132 Offenses are a bit hard especially with Koko suffers from good physical Electric-STAB options (everything bar Wild Charge is Special). Wild Charge and Thunderbolt both OHKO Aerodactyl if it can move before the Fossil Bird gets the chance to strike. Wild Charge also allows you to hit Snorlax very hard if your opponent thinks you are running a Special Set over a Physical Set seeing how Specially Defensive Snorlax can tank at least 5 T-Bolts and can set-up with Curse/Rest. If the Snorlax isn't running any Defense Investment, Life Orb Wild Charge 2-HKOs it if you hit it on the switch-in and on your following turn. If you run the IVs/EVs I mentioned and with Life Orb, Snorlax most likely won't last the turn after it's swapped out into the battle if it does decide to remain in. Either move is used for Koko's Electric-STAB although Wild Charge and Life Orb really hurts its' ability to last longer. T-Bolt allows Koko to survive longer but you get walled by Specially Defense opponents such as Specially Defensive Tapu Lele. Electro Ball seems odd but if you play supportive at +6 Sp.Atk and +6 Spd Electro Ball hits very hard and nearly OHKOs anything that doesn't both resist STAB Electric and carry an Assault Vest (Ground-Types are exempt since they are immune to Electric-Type attacks).
Brave Bird: If you saw that your opponent runs Tapu Bulu (one of the most powerful OU Pokemon in Sun/Moon given the 130 Base Attack it has) during the Pre-Battle start-up, the given IVs and EVs I mentioned allow Koko to do a guaranteed 3-HKO on the most physically defensive Bulu after Grassy Terrain recovery (no other recovery factored in) without Life Orb or a solid 2-HKO with Life Orb, you can easily 2-HKO Bulu if you can bait your opponent to switch Bulu in the turn you fire off the first Brave Bird and they keep it in when you use the second Brave Bird. If you see anything else worrysome, U-Turn out to something that can handle Bulu (if any). Be wary that Life Orb Brave Bird does a lot of recoil damage and Koko generally will not like that much recoil although Brave Bird is one of Koko's only answers to Grass-Type opponents who switch-in outside of HP Fire/Ice/Bug/Poison, U-Turn, Aerial Ace and Acrobatics.
Taunt/Nature's Madness/Calm Mind/Toxic/Grass Knot/HP Ice/Fire/Grass/Flying/Water: The last move is a bit of a toss-up. Taunt can be used against Stall Teams that switch in allowing you to disable their ability to set-up with buffs or status Koko making it easier to get around. Nature's Madness is a viable move, allows you to get rid of Blissey or other prominent Special Walls that switch-in on Koko making it easier for Koko or its' allies to net a faster KO. Calm Mind is a nice move if you run T-Bolt and/or Hidden Power since you boost both your Sp.Atk and Sp.Def at the same time making Koko a bit more bulky on the special side but if you can't afford to risk using it in the face of your opponent, best to not use it. Toxic, well, you can Toxic the Pokemon coming in (most likely a Ground/Grass/Electric or a Lightning Rod user) making it somewhat easier to KO them if they try to sweep without swapping out as you U-Turn out after hitting them with Toxic the turn they come in. Grass Knot is Koko's primary way of dealing with Ground-Type opponents who come in. Hits the likes of Hippowdon and normal Sturdy-Golem for a solid 2-HKO, hit them once on the switch in allows you to KO them if they stay in the following turn if they don't Protect. Hidden Power varies. HP Ice is notable for hitting common Ground responses such as Chomps, Mudsdale and Palossand as well as Bulu. Fire hits Grass-Types such as Bulu (again), Grass-Silvally and Dhelmise who could all come in by switching and tank-stall Koko. HP Grass hits Ground-Types that are common as well as but there aren't many Ground-Types that will be weak to Grass in the face of Koko outside of possible Mudsdale, Golem (both normal and Alolan), and Ground-Silvally. HP Water hits Fire, Ground and Rock-Type opponents hard if you get buffed by Baton Pass, HP Water allows you to hit Alolan Dugtrio pretty hard if the opponent thinks that you want to U-Turn first. HP Flying hits opposing Grass and Bug-Types for additional power although this isn't really needed unless you avoid using Brave Bird. HP Flying does OHKO 0 HP/0 Def Pheremosa and has a chance to OHKO 0 HP/0 Def Buzzwole when both are at max HP, given how popular the former is, that's impressive. Of course neutral Speed nature Pheremosa with 31 IVs and 252 EVs outspeeds positive Speed Nature Koko with 31 IVs and 252 EVs by 3 points. Still, Koko has that much of an edge while none of Pheremosa's strongest STAB options OHKO Koko at full HP unless Pheremosa's allies put up 3 layers of Spikes in addition to Stealth Rock (with this 252+ Atk Life Orb or 252+ Sp.Atk Life Orb Pheremosa has a chance to OHKO Koko after all that damage first if Koko was at full HP) but Koko still has the power OHKO the bug if she stays on the field even without entry hazard support.
Possible Allies (Pre-Poke Bank):
Slowbro: While slow (yes, it's obvious), Slowbro can be U-Turned into if your opponent swaps out for a Ground-Type Pokemon the turn you use U-Turn. Surf/Hydro Pump punished nearly all Sun/Moon Ground-Types for a lot of damage (I can easily say that Ground-Types in Sun/Moon are weak to Water-Type attacks so Slowbro helps out a lot here if you lack Bulu or Fini). Slowbro is also quite bulky itself meaning that it can Slack Off if it survives the switch-in.
Tapu Fini: A bit faster than most of the common Pokemon in Sun/Moon and another Fairy-Type. I don't know many Dragon or Ground-Types that will desire to stay in the battle once Fini comes in as its' Dual STABs can easily net a possible OHKO on them.
Tabu Bulu: A nice solid check and possible counter to Koko's weakness to Ground-Type attacks. Wood Hammer easily dents if not KOs common Ground-Type Pokemon with ease. Koko also can deal with Bulu's weakness to Flying-Type Pokemon.
Salazzle/Magmortor: The only two useful Fire-Type Pokemon that I can think of that you can get in Sun/Moon without needing to trade or Island Scan. The former is also part-Poison, so it can threaten off Grass-Types if it comes in. The former can also be used if you desire a Sunny Day with Solar Beam.
Grass and Water-Type Pokemon: In general, Grass and Water-Type Pokemon can help cover Koko when needed seeing how they can come in. Both types can deal with the Ground-Type opponents that would otherwise come in and wall/KO Koko if it lacked a U-Turn.
Zygarde: If Power Construct wasn't banned, 100% Zygarde is a wonderful helper as it can tank most of the hits that would threaten Koko thanks to 100% Zygarde's massive bulk. Normal Zygarde (50%) also helps if you really don't like searching for every Core and Cell.
Checks and Counters (Pre-Poke Bank):
Palossand: Palossand isn't OHKO'd by anything that Koko offers and easily KOs back with EQ or Earth Power, forcing Koko to U-Turn or be force-swapped.
Alolan Dugtrio: Another counter to Koko but only if the former lacks a positive Speed nature while the latter has a positive Speed nature. EQ OHKOs, otherwise +Speed Koko U-Turns out to a Grass or Flying-Type before you get a chance to hit. Best to not use a Choice Item since the opponent will possibly mess with you if they force-swap into a Flying-Type if you use EQ.
Tapu Bulu: A strong check to Koko as it resists the STAB Electric attacks and replaces the Electric Terrain for Grassy Terrain although it benefits both rather than just one of them like the Electric Terrain. Be wary that a Koko running the set I mentions 2-HKOs Bulu if you accidentally switch into a Brave Bird.
Zygarde: Regardless of Form, unless Koko is running Dazzling Gleam, you wall Koko as HP Ice does decent damage but you force it out as EQ/Land's Wrath OHKOs it.
Snorlax: Specially Defensive and bulky sets can wall Special Koko but you risk being 2-HKOd by mixed Life Orb Wild Charge Koko.
Mudsdale: Physical sets wall Koko as it can tank Brave Bird and HP Ice/Grass Knot while risky don't OHKO it. If you know that the opponent/guess that the opponent lacks them and will use U-Turn, you can swap out for a Fire/Ice/Flying-Type Pokemon to check the swapped-in Pokemon.
Garchomp: At most a possible 2-HKO by 252+ Life Orb Dazzling Gleam if not bulky. EQ or Earth Power OHKO Koko. Just don't send in Chompy if Koko got Baton Passed with Special Attack buffs. I learned that the hard way. Life Orb Brave Bird has sure-kill 3-HKO Chomps at full HP so don't let Chomps take hits before you send it in.
If anyone has any other ideas on what moves/items Koko could use or what allies help Koko that would be nice. I'm only taking Pre-Poke Bank Pokemon into consideration as I know what Post-Poke Bank Pokemon I will be seeing on teams just to keep Koko off the field. I'm keeping Koko at +Speed Nature since once Poke Bank works for Sun/Moon, Speed Tiers are going to mean quite a lot for non-Trick Room teams. Still working on getting another Koko with a different nature on my Moon File.