This topic may have a lot of grammatical errors, but hey, idea is an idea, right?
I looked at the comments and changed it a little bit, now it looks better and makes a little bit more sense (yet, might still have grammatical errors).

A while ago i had an idea of new guild hall decorations, the point is you use tranquil gun, sleep gas bomb, syringe weapon or sleep vial to put monsters in "sleep" and capture them with a chance of 5% when killed with one of above weapons or 1% when killed with stun weapon if non-construct and non-undead (living, if you want to call them that), 6% to capture construct enemy when killed with shock weapon and 2% to get undead if killed with stun weapon. Captured non-construct non-undead monsters will escape in 24 hours, construct monsters will rust and dissapear in 32 hours, and undead monsters will rot in 40 hours (or maybe become fertilizer if they add flowers into sk if not put in a cage or a pet house (guild hall decoration (purchased from birdsong emporium)).
Note: Only harmless monsters (look below for a little bit more pet house info) can be put into the pet house, they are not all friendly, you know.
Note: Captured monsters also can be purchased or sold on auction and maybe if three rings (sk creators) will want to, be bred at the lab.

Cages are used to keep classified monsters in them like this:
Small cage = 1 small/medium monster (jelly cube, lichen, zombie, gun puppy, polip, greaver, e.t.c.); Medium cage = 1 big monster (lumber, deadnaught, troyan), 4 small or 3 medium; Big cage = 2 big monsters, 9 small or 7 medium; Giant cage = 5(!!!) big monsters, 12 small or 10 medium; Pet house are used to keep harmless monsters (mewkat, silkwing, gunpuppy on wheels(haters gonna hate), and maybe some more harmless monsters if there arem or three rings add some). "Your pet" will be strolling around guild hall. Even though the monster will not be inside the pet house, it is required.
Note: You would be able to turn off/on "show GH pets" in game options.

Of course there are exceptions ( most can be ignored by crafting special cages with materials and normal cages),
you can't put in a cage: gremlin family monsters, phantom, mecha knigth series, retrode series, no type monsters, swarm inflicted monsters and bosses(both swarm inflicted bosses and not inflicted are absolutely uncatchable).
Special cages require boss materials, small require normal materials, specified token material and a small cage , medium and bigger cages require specified token material,specified shadow lair material, more normal materials and medium or bigger normal cage.
Note: Can be purchased or sold on auction, too.

I will try to make sleep weapons topic if this one gets hot (or at least add more details).