Poelala (MP)
Poelala is a must-have for all mages. As it derives its 'damage' from INT rather than CHA it does favor pure mages, but it's definitely very useful for Beastmasters too. I put the term 'damage' in quotation marks, because while it doesn't damage the monster at all, it instead boosts your magic weapon damage by 49.4%, and your spell damage by 18.5%.
Location: Valencia -> Golden Giftboxes -> UltraRare Shop
Dunamis (SP)
Dunamis is the best guest available for all warriors, though it favors pure builds due to the fact that it uses your STR rather than your CHA. Rather than damaging the monster, Dunamis instead boosts the player's melee damage by 37.1%.
Location: Valencia -> Golden Giftboxes -> UltraRare Shop
Thernda (SP)
Thernda works the same way Dunamis does, except it boosts ranged damage and uses the player's DEX. Ranged weapon attacks get a boost of 37.1%, and ranged SPells get half the boost: 18.5%. It's definitely worth noting however, that FD rangers should not use Thernda at all. Thernda in its base state is inferior to elemental guests, but as it derives most of its 'damage' from the player's boosts, such as misc items and the Chieftain's IronThorn shield , it ends up being greater when used with FO armors. Since FD rangers don't use the same boosts, they're much better off using normal guests.
Location: Valencia -> Golden Giftboxes -> UltraRare Shop
Memet (MP)
A solid Harm guest, though it lacks an MC and has a small damage penalty of 10%.
Location: Battleon » Warlic's Shop » Quests » Meet Memet
Grakma (SP)
Clicking on the Grakma Harbinger misc will call an extraordinary fire guest which deals 50% more damage at the cost of only attacking 2/3 of the time. Clicking on its tail will toggle between a pure-damage attack with a +10 BTH lean, and an attack which deals half the damage but inflicts Element Vulnerability (+50% damage, 1 round).
Location: Mt. Thrall -> Hollow's Garage -> I'm here to kick some demon tail!
Poutine Golem (SP)
A solid fire guest, though I'd recommend using Grakma over it. Its normal attack consists of 1 ranged hit, and it deals 75% damage. However, it also inflicts the Entangled status (-45 DEX) for the duration of the battle. If the monster already has the status, then the guest will instead perform a melee attack dealing 100% damage.
Location: Frostvale -> Chilly's Shop
Ghostly Infernal Skull (MP)
Toggles between pure damage and Afraid/Fear. Fear attack attempts to inflict a (55.5*FireRes*Hits/2)% Fear that lasts for 1 turn.
Location: Valencia -> Golden Giftboxes -> Rare Shop
Infernal Djinn (MP)
Equipping the Infernal Djinn Lamp misc will summon a powerful guest which seeks between fire and darkness.
Location: Warlic's Shop -> Buy Magic Equipment -> Expert Equipment
Aquella (MP/SEASONAL)
A solid water guest whose attacks can Daze the monster.
Location: SnuggleFest -> Season of Shipping (SnuggleFest 2018)
Gaiden (MP)
Click to swap between pure damage and healing.
Location: The Bizarre Flecks Saga -> Part 5 - Us vs. Regeirk!
Rhubzard (SP)
Location: The Void
Chupacabra (SP)
A great guest that compresses water and dark, and seeks between the two. The darkness attack consists of 2 hits, and the water attack consists of 3 hits.
Location: Battle Monsters -> Dead Goat -> Follow Chupacabra
Maple Beaver (SP)
Maple Beaver is a solid guest that compresses water and earth, and seeks between the two. Its attack is one melee hit.
Location: Guardian Tower -> Guardian Account Shop
Skeeter (HP)
Has a 50% chance of doing 1 ranged hit, 80% damage, and a 50% chance of doing 3 melee hits, 120% damage. In addition, it uses a very small amount of HP (only 16 every turn) for its upkeep, rather than your SP, making it extremely useful.
Location: Zard Hunter's Shop -> Frogzard Hunter -> Part 3
Eukara Vox (MP)
Location: Robina's Shop -> Truphma Saga -> 1: Introductions -> Truphma World Jumping!
Wind Spirit (MP)
A solid option for wind. It has an 80% chance of doing 2 hits, 75% damage, and a 20% chance of doing 2 hits, 200% damage.
Location: Bridge to the Sky -> Imperial Regalia
Gemini Monks (SP)
Equipping the Gemini Pendant misc will call a strong guest that seeks between wind and earth. Both the wind and earth attacks consist of 1 melee hit each.
Location: Guardian Tower -> Guardian Shop
Nezujimbo (SP)
Location: Travel Map -> Crossroads -> Farmlands -> BURP War 2012
Mirror Ryuusei (MP)
Location: Bizarre Flecks -> 2: Meteor Storm
Icemaster Yeti (MP)
A decent option for ice. Its normal attack consists of 1 hit, and has a high chance of Dazing the monster; however, it deals 50% less damage to compensate for this. If the monster is already dazed however, then the guest will instead perform a very powerful 1-hit melee attack. Both attacks have a -10 BTH lean, and thus deal 13% more damage. It works especially well when paired with the Ice Cream Golem pet.
Location: Valencia -> Golden Giftboxes -> Common Shop
Sugar Gobbler (SP/SEASONAL)
Location: Mogloween -> Candy Caverns
Doomquake Minions (MP/Z-TOKEN)
Scorp - Accurate Earth guest. Click to switch between a pure damage attack and an attack that inflicts Poison. Rast - Inaccurate Earth guest. Click to switch between a pure damage attack and an attack that inflicts Bleed. You can have both guests out at the same time and you pay both of their MP upkeeps.
Location: Travel Map -> Sail East -> DoomQuake Cavern
Gemini Monks (SP)
Already mentioned in the wind section.
Location: Guardian Tower -> Guardian Shop
Maple Beaver (SP)
Already mentioned in the water section.
Location: Guardian Tower -> Guardian Account Shop
Mogdin (SP/MP/RARE)
Location: None
Guano Loco (MP/SEASONAL)
Loco is a brilliant, although unreliable and inconsistent guest, that returns every year on April 1st. Its first attack has a 69% chance of occurring, and does 10% damage. The second attack has a 25% chance of happening, and does 200% damage. The last attack only has a 6% chance of occurring, but deals 690% damage.
Location: April Fools -> 2014
EnergyZard (SP)
If you don't have either of the above guests, then your only options left are EnergyZard or StormZard depending on your build. EnergyZard has as a 50% chance of doing 1 melee hit and a 50% chance of doing 2 ranged hits, with both attacks being equally powerful.
Location: Zard Hunter's Shop -> Frogzard Hunter -> Part 1
StormZard (MP)
Has a 50% chance of doing 1 melee hit, 66% damage, and a 50% chance of doing 2 ranged hits, 133% damage.
Location: Zard Hunter's Shop -> Frogzard Hunter -> Part 3
Artix (MP)
Clicking on the Paladin's Oath misc will summon Artix, a powerful light guest with an MP upkeep. He deals increased damage against undead monsters, but ever so slightly lower damage against all else. He can also heal you if you're low on HP.
Location: Yulgar's Inn -> Advanced Shop -> Shields
RayZard (SP)
Sadly the only non-rare option for an SP light guest. It has a 50% chance of doing 1 melee hit and a 50% chance of doing 2 ranged hits, with both attacks being equally powerful and dealing 100% damage.
Location: Zard Hunter's Shop -> Frogzard Hunter -> Part 2
Ninja Lawyer Assassin Contract (SP/SEASONAL)
This is the best option in the game, returning every year during the annual Blarney War. It has 3 attacks, and you have to pay a small amount of gold in order for the guest to use them. The first attack deals 65% damage and Entangles the monster, lowering their DEX by 90. If the attack is repeated then the effect stacks additively, but note that the monster's DEX cannot fall below -500. The second attack consists of 1 melee hit, has a +5 BTH lean and deals 60% damage. In addition, it can cause the monster to bleed. The third attack consists of 3 melee hits, has a +10 BTH lean and deals 140% damage.
Location: Blarney War
Summon Undead Army (MP)
A solid option for darkness. The guest has two attacks: one melee, and one both ranged and magic. The former has a +5% damage MC, and the latter has a +4.25 BTH MC. Both attacks have an equal chance of occurring.
Location: Rip in the Sky -> Adder's Forge
Chupacabra (SP)
Already mentioned in the water section.
Location: Battle Monsters -> Dead Goat -> Follow Chupacabra
Infernal Djinn (MP)
Already mentioned in the water section.
Location: Warlic's Shop -> Buy Magic Equipment -> Expert Equipment
Grimlord of Nulgath
A solid dark guest for damage. Pays HP cost each turn to gain +100% damage. It attacks normally for 4 hits, and also has a mastercraft attack that attempts to paralyze the enemy. The special attack is 2 hits and has a 9.9% proc rate. Monsters can save against the effect using their END and LUK vs your CHA and LUK, with a +20 bonus.
Location: Valencia -> Golden Giftboxes -> Rare Shop