I have to confess that I'm not really enamoured of 'Monsters' in the classical sense. The Monsters in Rune Factory are quite different as they can be charming and cute. The Chipsqueeks immediately come to mind. In FarmVille, however, Monsters tend to be hideous and frightening in the classical way that Monsters have been portrayed throughout the ages.
Despite this, one must complete the Monster Lab in the new Haunted Hollow Farm if one intends to succeed in the Spooky Quests... and I therefore did so and am trying to unlock the Monsters in the Lab by using the Serum that one can harvest periodically from the Lab.
The reason I am creating this post is because I could not obtain anything but the Blue Serum for ages from the Lab. I now realise rather belatedly that the Monster Lab was created on the same principles as the Unicorn Island. In other words, in the same way that the Purple Amethyst is the default Gem harvested from Unicorn Island (and will be the one that is harvested from all existing Unicorns placed there as well as those who correspond to that Gem), the Blue Serum in the Monster Lab may be the default. It certainly is the Serum that corresponds to the original Chickenpire Monster you create.
The first Monster created in the Lab is the Chickenpire Monster and it is the one that gives Red Serum from the start.
I have unlocked very few but here are the correspondences I have so far:
Chickenpire: Blue Serum
To create: 4 Blue Serum
Frankensheep: Green Serum
To create: 5 Blue Serum
Monster Hog: Blue Serum
To create: 2 Blue Serum, 4 Green Serum
Mummy Duck: Orange Serum
To create: 4 Blue Serum, 3 Green Serum
La Chupacabra: Yellow Serum
To create: 4 Blue Serum, 3 Green Serum, 2 Orange Serum
Giant Moth: Orange Serum
To create: 4 Blue Serum, 5 Green Serum, 4 Orange Serum, 2 Yellow Serum
Fish Creature: Pink Serum
To create: 5 Blue Serum, 7 Green Serum, 6 Orange Serum, 6 Yellow Serum
Mega Monkey: Yellow Serum
To create: 6 Blue Serum, 4 Green Serum, 10 Orange Serum, 6 Yellow Serum, 4 Pink Serum
To create: 6 Blue Serum, 8 Green Serum, 6 Orange Serum, 10 Yellow Serum, 8 Pink Serum
In other words, it probably is best to keep your Monster Hog out of the Monster Pen and simply Harvest from the Pen containing nothing more than the Frankensheep until you have sufficient Green Serum to unlock and create the fourth Monster in the form of the Mummy Duck. I daresay you then will need some Orange Serum from the Mummy Duck for the fifth Monster.
As I confessed, I haven't made any real haste to move forward on this, quite unlike my actions with respect to the Unicorn Island, where I unlocked all five pages of Unicorns as quickly as possible. If a Player is as keen on Monsters as I am on Unicorns, he/she can build a second Monster Pen to be able to harvest more Serum... Another method of creating more Green Serum is by creating another Frankensheep. In the same way that a new Unicorn will generate a Gem upon creation, a new Monster generates its corresponding Serum colour.
If you would like to obtain any Harvests of Green Serum, you should remove the Chickenpire from the Lab with all haste after creating the Frankensheep.
If you look at the list of ingredients required to create the first six Monsters, you will see that as you progress, more Serum of EVERY colour is needed. As with the Unicorns, you will discover that the Blue and Green Serum you formerly spurned must be created in quantity if you wish to continue to unlock new Monsters. Again, the quickest way to achieve this is by having more than one Monster Pen and placing a single Monster for each colour in each Pen.
Remember that Monsters like the Imp, a Quest Reward and a Creature that must be Mastered in order to complete one of the Quest Sequences, will yield NO Serum. Only Monsters created in the Pen will yield Serum. There is no advantage to an increase in the number of any single type of Monster placed in your Pen. One Giant Moth will yield as much Orange Serum as three Giant Moths. You can harvest the Monster Pen twice daily, however.
Incidentally, Mastery of specific Monsters and harvesting the Monster Pen a specific number of times both have been requirements in Quest Sequences. Furthermore, the creation of ALL Monsters in the List is a Requirement for 'beating' the Haunted Hollow Farm. When you 'beat' the Haunted Hollow, you will obtain a Shipping Licence that will allow you to share items freely between the Haunted Hollow and any other Farms for which you have Licences as well as giving you a Statue to commemorate the achievement.