Useful Items in Monster Rancher 2
There are many items in Monster Rancher 2 that aren't mentioned on this page, this page selectively chooses the few items in the game that are most beneficial and talks about them. The rest of the items aren't useless by any means but they're largely ignorable compared to the ones on this page. If you want to raise a strong and healthy Monster; you need to be very familiar with these specific items. If you'd like to see what all the items do I recommend the Monster Rancher 2 FAQ on Gamefaq.
Magic Bananas
Until you've made a lot of money in Monster Rancher 2 and are able to afford raising a Monster to the best of your abilities I recommend ignoring the Magic Bananas. The only reason to use this item is to increase your Monster's lifespan and unless you're already doing everything in your power to make the Monster as strong as it can possibly be during its short life, they aren't worth using.
How the Magic Banana item works is very straight forward, it sometimes does nothing, it sometimes increases Loyalty and sometimes decreases it. It's largely ambigious what else it does but thanks to some nerds on the internet and experimentation there's plenty of math proving that it increases a Monster lifespan when it deducts their Loyalty. It also does the opposite when it adds Loyalty, it'll decrease their lifespan.
-10 Loyalty; +1 week lifespan
-5 Loyalty; +1 week lifespan
+10 Loyalty; -1 week lifespan
+5 Loyalty; -1 week lifespan
Mint Leaves
Stress reduced by 50%; -5 Form and -2 Spoil
The Mint Leaves are the most important consumable item in all of Monster Rancher 2. When you raise a monster in this game you can fully recover your fatigue by Resting but you can't fully recover your stress this way. For even the most relaxing training methods you'll need to use Mint Leaves unless you plan to Battle with your monster monthly, then you probably won't need Mint Leaves as much... However battling monthly will greatly gimp your monster in terms of stat gains.
Nut Oils
-28 Fatigue; +1 Spoil; +1 Fear (If your Monster likes Nuts Oil it'll also give you -20% Stress)
This item is required for rigorous training regimens but otherwise can largely be ignored. If you use a Nut Oil every other week you usually won't need to Rest a monster, instead you'll be able to run one or two Hard Drills a week and the rest Light Drills. It's a very important item to anyone who wants to min/max their monster.
Tablets (Monthly Food)
Liked by Monsters: -15 Stress; +3 Spoil; +5 Form
Neutral with Monsters: -13 Stress; +1 Spoil; +3 Form
Disliked by Monsters: -10 Stress; -1 Spoil; +2 Form
Without a doubt in the world, Tablets are the best food for you to feed your monster each month. Even if they dislike Tablets, they're still the best food to feed your monster; even when your monster is a baby, Tablets are still the best food. The only reason you should ever feed them a different food is if you're trying to save money or accomplish a task such as the Cup of Jelly Diamonds for Ducken.
Fire Stone
This item increases the effects of Rest for your Monsters; increasing how much Fatigue and Stress it takes off from your Monster very slightly. The Stress and Fatigue loss only occurs during the months of November, December and January.
Fire Stone is found in the Kawrea Expedition which you're asked to go to on June 1st, assuming you have a high ranking monster.
Lump of Ice
This item increases the effects of Rest for your Monsters; increasing how much Fatigue and Stress it takes off from your Monster very slightly. The Stress and Fatigue loss only occurs during the months of May, June and July.
Lump of Ice is found during the Torles Expedition which you're asked to attend on Feb 1st, assuming you have a high ranking monster.
Gold Peach
This is one of the rarest items in the entire game and definitely the best. When you consume a Gold Peach you will increase your Monster's lifespan by 50 weeks. You can only consume a single Gold Peach in a Monster's entire life though, which totally sucks.
Gold Peaches are found in the Tusk Tower during the Parepare Adventure. This tower is found in the central portion of the adventure area, just north of the lake and nearby the large destroyable pumpkin. You can see its exact location and what it looks like in my screen shot below.
Silver Peach
The second item that increases your monster's lifespan in Monster Rancher 2 is a Silver Peach. This item can only be acquired by participating in the Elders' Cup, a tournament which happens once a year in the second week of June. If your monster is under the age of 6yr 3mo you won't be able to participate in the Elders' Cup, it actually won't even show up in your list of available tournaments during this time.
Consuming a Silver Peach increases your monster's lifespan by 25weeks. You're able to feed your monster both a Gold Peach and a Silver Peach but not two of the same kind of peach. Aside from these two peaches and Magic Bananas there's no other way to extend your monster's life.
Metalner Disc Chips
The screen shot above is of Metalner Disc Chips but I am going to talk about all Disc Chips. They're a rather valuable item, most sell for 500 G a piece and some go for 1k or even 2k gold. Other than being nice vendor trash, the Disc Chips will also grant you a bonus when using them during combination.
This bonus changes depending on what type of Disc Chip you're using, for example the Metalner Disc Chips shown above give your monster fear +50. If you use Naga Disc Chips you'll get +50 skill; Ape Disc Chips will grant you a +1 bonus to all stats on the Parepare Errantry.
Mock and Plant have arguably the best bonuses since they'll give your monsters and extra 10 weeks added to their life spans! I like those two as well as any Disc Chip which grants you a bonus to certain drills (typically I pick a stat my monster is weak in). Some drills you run almost 100 times during a monster's lifespan, a +1 bonus will help out a lot in those situations!
Ape: Parepare Errantry grants +1 bonus to stats
Arrowhead: Uses Guard when low on HP during Battle
Bajarl: Uses Vigor during Battle
Baku: + 50 to Fat or Skinny characteristic
Beaclon: Pull Drills grant +1 bonus to stats
Centaur: Mandy Errantry grants +1 bonus to stats
Color Panda: +50 to spoil
Dragon: Uses Fury during Battle
Ducken: +50 Speed
Durahan: Domino Drill grants +1 bonus to stats
Gaboo: Uses Fight during Battle
Gali: +50 to Good or Bad characteristic
Ghost: Dodge Drills grant +1 bonus to stats
Golem: +50 Power
Hare: Uses Grit during Battle
Henger: Shoot Drills grant +1 bonus to stats
Hopper: +50 to Fat or Skinny characteristic
Jell: Endure Drills grant +1 bonus to stats
Jill: Papas Errantry grants +1 bonus to stats
Joker: -50 to Good or Bad characteristic
Kato: Meditate Drill grants +1 bonus to stats
Metalner: +50 Fear characteristic
Mew: Uses Hurry during Battle
Mocchi: +50 Popularity characteristic
Mock: 10 weeks added to monster's life span
Monol: +50 Defense
Naga: +50 Skill
Niton: Swimming Drill grants +1 bonus to stats
Phoenix: Kawrea Errantry grants +1 bonus to stats
Pixie: +50 Intelligence
Plant: 10 weeks added to a monster's life span
Suezo: Uses Ease during Battle
Tiger: Uses Will during Battle
Undine: Study Drill grants +1 bonus to stats
Worm: +50 Life
Wracky: Leap Drill grants +1 bonus to stats
Zilla: Torble Sea Errantry grants +1 bonus to stats
Zuum: Run Drills grant +1 bonus to stats