Other Options

Giga Drain is Muk's most viable other option, enabling it to hit Rhydon and Golem for massive damage. Since Muk is faster than both of these Pokemon, it can even force them out if they're hit on the switch; it also hits Cloyster super effectively. However, without Curse, Muk will not be able to pressure Pokemon like Snorlax, Zapdos, Raikou, Suicune, Misdreavus, Vaporeon and even Psychic-less Starmie nearly as effectively. While Giga Drain alleviates pressure from the likes of Golem and Rhydon and can even serve to bait in these Pokemon, it can make Muk significantly less dangerous overall and force it to use Explosion to go for a trade much earlier in the game.

Besides Giga Drain, Muk doesn't have many other viable options. Dynamic Punch could be used to bait Tyranitar, other Rock-types, and also deal significant damage to Snorlax. Zap Cannon and Thunder can be used to spread paralysis and hit Skarmory and Cloyster, but other Pokemon are significantly better at doing this, and Fire Blast is much more consistent, especially against Forretress and Steelix. Thief can be used with no item on hand to steal a foe's item, but the loss of a coverage move or Curse is almost never worth it.

Checks and Counters

Rock- and Ground-types: Tyranitar, Rhydon, Golem, Nidoking, and Marowak resist Sludge Bomb, shrug off Fire Blast, and can all threaten Muk with a powerful Earthquake; Rhydon, Golem, and Tyranitar additionally resist Explosion, but they would hate to get poisoned unless they carry Rest. If Tyranitar doesn't carry Earthquake, it might not be immediately threatening against Muk, which could stay in and try to inflict poison with Sludge Bomb. Rhydon and Golem resist Explosion, but they don't want to take a Giga Drain in the rare case that Muk is carrying it. Marowak and Nidoking need to watch out for Explosion, and Muk can also inflict poison on the former. There are also a few niche defensive Rock-types that can check Muk quite well or at worst trade with it. Shuckle, Omastar, and Aerodactyl shrug off Muk's common attacks, resist Explosion, and carry Rest to cure poison from Sludge Bomb.

Steel-types: Skarmory and Steelix resist Explosion and are completely immune to Sludge Bomb. They need to watch out for Fire Blast, but can take one hit from it if necessary. While Steel-types don't check Muk well overall, they can still be very useful due to their Poison immunity and resistance to Explosion.

Ghost-types: Gengar and Misdreavus are immune to Explosion and shrug off all of Muk's other attacks, although Sludge Bomb can poison Misdreavus. Gengar and Misdreavus can threaten Muk with Hypnosis and even a combination of Mean Look and Perish Song, but Perish trap Misdreavus should only check Muk as a last resort, since it doesn't want to be poisoned. Without these moves, Misdreavus will struggle versus Muk because a +1 Sludge Bomb from Muk does more damage to Misdreavus than a Psychic does back to Muk. Gengar's Thunder does about as much damage to Muk as its Fire Blast does back, so this matchup without earlier-mentioned moves is about even until they trade when Gengar decides to use Explosion, assuming neither Pokemon switches out.

Psychic-types: Exeggutor and Jynx can hit Muk with a strong Psychic or put it to sleep with Sleep Powder and Lovely Kiss. Neither of these Pokemon wants to take a Sludge Bomb or Explosion, however, so they must be careful when trying to check Muk. Starmie can switch into Muk relatively well and can 2HKO it with Psychic after Spikes while not minding Poison that much thanks to Recover; however, it risks trading itself with Muk, as Muk can often be paired with sweepers that want Starmie gone, like Vaporeon. Overall, Psychic-types can offensively threaten Muk but none can switch in safely.

Bulky Special Attackers: Zapdos, Raikou, and Suicune can use Rest to cure poison from Sludge Bomb and wear down Muk to the point at which it will either use Explosion or be easily revenge killed by another teammate. That said, this can often be to the Muk user's advantage, as it often aims to trade with these Pokemon. Only use Zapdos, Raikou, and Suicune to check Muk if you don't mind trading with it or if you have no other checks on your team.

Umbreon: RestTalk Umbreon can reduce Muk's Attack with Charm and check it quite well; however, it risks trading itself for Muk if the Muk user decides to use Explosion as Umbreon switches in.

Cloyster: Cloyster has very high Defense, so it can take advantage of Muk by setting up Spikes early-game; however, Muk could use Cloyster to set up Curse, potentially forcing it to use Explosion. Although Cloyster's Explosion doesn't OHKO Muk after one Curse boost, it still deals a large amount of damage.