Ever wanted an ally like you would fight in the Clockworks, but you are forced into Seraphynx when you wanted a Spookat? Well, then a Monster Friend would become your best hope! First, a TL;DR.
TL;DR: Monster Friends that require luck to obtain, with special methods of obtaining, PLUS a one-time event that gives the suggester and some preset and random winners who get a Vog Cub, plus anyone can get a Monster Friend then easy.
You would have to rely on LUCK to get a chance to capture one. Then more luck required to capture it successfully. What you can get as a Monster Friend:
-Wolver series (Valestone)
--Wolver (Piercing)
--Frostifur (Freeze/Elemental/Piercing)
--Voltail (Shock/Elemental/Piercing)
--Ash Tail (Fire/Elemental/Piercing)
--Rabid Wolver (Poison/Piercing)
--Void Wolver (Piercing/Shadow)(Dark Matter)
--Vog Cub (Fire/Elemental/Piercing)(Winner)
-Chromalisk series (Valestone)
--Chromalisk (Normal)
--Salamander (Fire/Elemental/Normal)
--Tundralisk (Freeze/Elemental/Piercing)
--Electrolisk (Shock/Elemental/Normal)
-Devilite series (Pep Talk)
--Devil-IT (Shock/Elemental/Shadow)
--Devilite Overtimer (Stun/Piercing/Shadow)
--Devil-IT Overtimer (Shock/Stun/Piercing/Elemental/Shadow)
-Lichen series (Dark Matter)
--Void Gel (Poison/Normal/Shadow)
-Thwacker series (Dark Matter)
--Void Thwacker (Piercing/Shadow)
-Lumber series (Moonstone)
--Lumber (Stun/Elemental)
--Redward (Fire/Stun/Elemental)
--Void Lumber (Stun/Piercing/Shadow)(Dark Matter)
-Retrode (Moonstone)
--Retrode (Laser Fire/Elemental)
--Voltrode (Laser Fire/Shock/Elemental)
Plus a one-time event called......... The Clockwork Master Event. Monsterbrine, Bionoman-Richard, Japanese-fire-dragon, Epic-Spy-Dude, Every Known Guild Master In Existence, and some random winners get a free Vog Cub with a Energy-decorated collar (White for preset winners, blue for random winners), plus everyone who tames a Wolver/Chromalisk during this time gets a spiked collar on their pet (White: set winners, Blue: non-set winners, Purple: everyone else, Gold Spikes: Spiral Knights Team/set winners), and Spiral Knights Team and the preset winners get Energy Spiked Collars on any pets they tame afterward, while the random winners that got two or more Monster Friends get, on random, Crimsonite or Luminite Spiked Collars and the non-Spiral Knights Team people who got three or more Monster Friends get the other Spiked Collars (Valestone: 3 or more Monster Friends, with no need for any one of them to be Construct or Overtimer or Swarm-Infected ones, Moonstone: 5 or more Monster Friends with 1 or more Constructs required, Dark Matter: 7 or more Monster Friends with 1 or more Constructs and 1 or more of either Fiend or Swarm required) for their post-participation reward.
Monster Friends
Understand that lumbers would have a boost in health against weapons. They would also take only 150.0% Elemental damage, and 67.5% Piercing damage. And the Swarm ones would need to be isolated in "survival pockets" where the Swarm will do low damage to you, but you can bring a Swarm-Infected monster in to begin the randomization. Plus Void Thwackers are technically no longer a true Gremlin. And do I need to remind you about Emberlight? And the chance is based on the health needed to otherwise defeat them. Not everything can be tamed. Read the list. Gorgos give more abilities than Devil-ITs. The health of the very strong ones would be upped to make it harder on you to defeat them, as Monster Friends are something that comes where defeat normally would.
I think having lumbers may be a bit overpowered, and you can't really have a gremlin pet because they're basically pitted against you from the beginning. Maybe you could try to make increased chances for obtaining friends with weaker/smaller enemies (eg chromas, gorgos, scuttlebots, minis etc). While with the harder ones (eg wolvers and normal devilites) the chances should be lower because they offer more combat abilities. While for lumbers/trogans or whatever, they're way too OP as "friends".