This page lists all Metal Slug Defense events and versions that have awarded exclusive units. These events are or were accessed via the Event button at the bottom of the screen in Sortie mode. Most of these events also awarded bonus medals for beating and/or S-Ranking the stages within.
Events are version-specific and will remain active as long as the player's game is still of the relevant version(s). For instance, a player will still have access to the Dai-Manji event as long as they do not update their game past version 1.24.0. It is important to disable 'Automatically Update Apps' if the player wants to stay on an older version even when the game has been officially updated.
Version 1.23.0-1.24.1: 1st Anniversary Event[]
Dai-Manji: Rescue all POWs (General Morden in his underwear) in all areas of the boss stage.
Version 1.28.0-???[]
Gen.Morden ver. Di-Cokka: Log in on Eleven different days. 20 Medals are awarded on the first 10 log-ins as well. Although this unit did not come from levels under the Event button, it is no longer available in current versions (until the Survival Events made many past units available).
Version 1.29.0-1.30.0: MSD MAX Event[]
Regular Army Helicopter: Rescue all POWs in all areas of the boss stage.
Version 1.31.0: Midsummer Horror Night Event[]
Zombie Eri: Rescue all POWs in all areas of the first stage.
Zombie Fio: Rescue all POWs in all areas of the second stage.
Zombie Tarma: Rescue all POWs in all areas of the third stage.
Tar Man(Red): Rescue all POWs in all areas of the boss stage.
Version 1.32.0-1.33.1: 25 Million Downloads Commemorative Event[]
Slug Gigant: Find a machine part in all stages up to boss stage (not including the boss stage).
Rebel Gigant: Beat the boss stage to unlock in the shop. The boss only takes damage from Slug Gigant or Metal Slug Attacks (minimal). Costs 500 Medals. The Rebel Gigant stays in the shop even if the player updates to 1.34.2 and beyond, as long as it has been unlocked.
Version 1.34.2: Battle Cats Collaboration Event[]
Cat: Rescue the POW in the first boss stage.
Black Cat: Rescue the POW in the second boss stage
The following units are available in the shop:
Wall Cat: 300 
Gross Cat: 300
Titan Cat: 300
Teacher Bun Bun: 400
Black Wall Cat: 300
Black Gross Cat: 300
Black Titan Cat: 300
Bun Bun Black: 500
Version 1.35.0-1.35.1: Battle Cats Collaboration Event part 2[]
Cat Storage: Rescue the POW in the third boss stage (map 1)
Cat Infantery [sic]: Beat the fourth boss stage (map 2) to unlock in the shop for 300 .
Cat Mini UFO: Beat the fifth boss stage (map 2) to unlock in the shop for 300 .
Iron Nokameow: Beat the sixth boss stage (map 2) to unlock in the shop for 500 .
Cat Slug: Rescue the POW in the seventh boss stage (map 2) (free)
Note: Unlike Rebel Gigant, which remains in the shop after being unlocked but not purchased, all cat units, including those that had to be unlocked before being purchased (e.g. Iron Nokameow), were removed in 1.36.1.
Version 1.36.1: Halloween Area[]
Version 1.36.1 released a new area on Alternate World Map 3 marked by a red exclamation mark (!). The Midsummer Horror Night event units were made available again, but this time as purchasable units after beating certain levels. This was not an event, but the stage has been changed to an ordinary area (Area 9) in version 1.37.0. Whether or not these units are still unlockable via this stage in 1.37.0 is unclear.
Zombies Eri, Fio, and Tarma: Unlocked in the shop for 150, 200 and 250 respectively after beating stages 1, 2 and 3 respectively.
Tar Man(Red): Unlocked in the shop for 300 after beating stage 4.
Halloween Soldier: Unlocked in the shop (0 , but players still had to visit the shop and 'purchase' it) after beating Stage 5 (Hozmi stage).
Version 1.37.0: Survival Event "Operation Round-up in the Night"[]
Nearly all previous event units are now available for purchase with Coins in the Event map base shop, except for Slug Gigant, the cat units, and Halloween Soldier (unconfirmed). Two new units are also available:
Allen O'Neil(Wrath): 1,000,000 Coins
Sand Marine: 2,000,000 Coins
Past units:
Zombie Eri: 25,000 Coins
Zombie Fio: 50,000 Coins
Zombie Tarma: 100,000 Coins
Tar Man(Red): 150,000 Coins
Dai-Manji: 250,000 Coins
Regular Army Helicopter: 500,000 Coins
Gen.Morden Ver.Di-Cokka: 750,000 Coins
Version 1.38.0: Amadeus's [sic] Attack[]
An extension of the Survival Event with a new level containing five new stages. The units from Version 1.37.0 are still available.
Amadeus Robot: 100,000 Coins
Researcher: 100,000 Coins
Unlocked after beating Stage 5-5:
Amadeus Type:α: 2,500,000 Coins
Amadeus Type:β: 2,500,000 Coins
Additionally, for those who missed the 1.32.0 event:
Slug Gigant: 2,000,000 Coins
Version 1.39.0: Christmas Event[]
The previous Event stages have been removed and replaced by five new sets of stages. The format of the event is similar, with stars as the currency in place of coins. The player loses all their coins from the previous version upon updating. Amadeus Robot and Researcher are no longer available in the event shop, but the bosses remain provided they were unlocked, although they are now bought with stars instead.
New units in the Event Shop:
Balor: 25,000 Stars
Formor: 50,000 Stars
Beating Event Stage 5-4 unlocks the following in the shop:
Cabrakan: 1,000,000 Stars
The following units are randomly unlocked when playing the event stages from Area 3 onwards. The last snowman the player needs is unusually difficult to unlock, and is usually Snowman Tarma:
Snowwoman Eri: 500 Stars
Snowwoman Nadia: 500 Stars
Snowman Trevor: 500 Stars
Snowwoman Fio: 500 Stars
Snowman Marco: 500 Stars
Snowman Tarma: 500 Stars
In addition, two new units are available in the main shop, and are exclusive to this version:
Christmas Eri: 300
Christmas Fio: 300
Version 1.40.0: New Collaboration Event[]
Invitations to fight Rugal Bernstein now drop in the 'KOF Stages', the level marked by the KOF symbol in Another World Map 3. The invitation drops in the same fashion as the Gas/Ammo Box/Card/POWs i.e. from hidden sweetspots on the map. (Use Samurai Planes to find it and reset the game if it doesn't drop)
Finding the invitation unlocks the fifth stage of the KOF area. This stage costs 60 sortie and has a version of Rugal Bernstein that is immune to knockback, automatically attacks air units with melee, and has an extremely short charge time for his special. Upon defeat, the following is unlocked in the main shop:
Rugal Bernstein: 500 (250 with the ongoing sale)
The player is awarded 14 Medals for beating this stage. Losing to Rugal locks the stage and the player has to find an invitation again.
Christmas Eri and Christmas Fio have been removed from the main shop, but the survival mode from the Christmas Event is still available.
Version 1.41.1: Cooperation Event[]
1.41.1 introduces a new kind of challenge where players have to cooperate with others to clear stages. Once again, any points from the Christmas Event are erased. Points are earned in the cooperation event in the form of 'combo medals' (represented by the 'combo' item from Metal Slug 4). (unofficial name) The player is given a special rank for the event based on the number of points they have accumulated. This does not affect their PvP rank.
Most of the past event units are still available for purchase with the new currency, except for 'unlockable' units like Cabrakan, Amadeus types etc. if they have't been unlocked. Rugal is still available in the main shop if unlocked, but will be at full price (500 ).
New units in the Event shop:
Kraken: 5,000 points
When the player reaches 10,000 points, their rank is replaced by the NPC icon, which is actually the Super Devil icon from the original Metal Slug. The following units are unlocked in the shop:
Super Devil (Marco): 10,000 points
Super Devil (Tarna): 10,000 points
Super Devil (Eri): 10,000 points
Super Fevil (Fio): 10,000 points
(Note: they all share the name 'Super Devil', without the protagonists' names after that. This is presumably SNK's method of preventing the player from equipping more than one in their deck.
Party People: 10
Party People Cadre: 30
Party People Elite: 90
Version 1.42.0: Cooperation Event part II[]
A continuation from the previous event. No new items are in the event shop, and all old units are still available. These new units are unlocked by S-ranking all the stages and getting 2,000 (fresh) points from each area.
Walter Rian: Area 1 (Blighted Street)
Tyra Elson: Area 2 (Construction Site)
Gimlet: Area 3 (Collapsed Highway)
Red Eye: Area 4 (Subway)
Emain Macha: Area 5 (Morden's Armies Base)
A new exclusive unit was also released in the shop:
Valentine Nadia: 300