Answer: CHARIZARD
Explanation: D/P Pokédex entry: 'It is said that CHARIZARD's fire burns hotter if it has experienced harsh battles.'
Answer: MEDICHAM
Explanation: Diamond Pokédex entry: 'Through yoga training, it has honed its sixth sense. Its movements are elegant.' The reference is obviously to the movie The Sixth Sense.
Answer: PHIONE
Explanation: D/P Pokédex entry: 'A Pokémon that lives in warm seas. It inflates the flotation sac on its head to drift and search for food.'
Answer: AMBIPOM
Explanation: Pearl Pokédex entry: 'They work in large colonies and make rings by linking their tails, apparently in friendship.'
Answer: TYROGUE
Explanation: Emerald Pokédex entry: 'Tyrogue become stressed out if they do not get to train every day. When raising this Pokémon, the trainer must establish a regular training schedule.'
Answer: DELIBIRD
Explanation: Gold Pokédex entry: 'It carries food all day long. There are tales about lost people who were saved by the food it had.' Kind of like a St. Bernard dog with a barrel of brandy around its neck, except a Delibird and with food in a bag.
Answer: NINCADA
Explanation: FR/LG Pokédex entry: 'Because it lived almost entirely underground, it is nearly blind. It uses its antennae instead.'
Answer: LICKILICKY
Explanation: D/P Pokédex entry: 'It wraps things with its extensible tongue. Getting too close to it will leave you soaked with drool.'
Answer: BUNEARY
Explanation: Diamond Pokédex entry: 'It slams foes by sharply uncoiling its rolled ears. It stings enough to make a grown-up cry in pain.'
Answer: JIGGLYPUFF
Explanation: Sapphire Pokédex entry: 'When this Pokémon sings, it never pauses to breathe. If it is in a battle against an opponent that does not easily fall asleep, Jigglypuff cannot breathe, endangering its life.' The Insomnia ability causes it to be impossible for the Pokémon to fall asleep, so then I guess Jigglypuff will just die.
Answer: TOGEKISS
Explanation: D/P Pokédex entry: 'It will never appear where there is strife. Its sightings have become rare recently.' But... how can it avoid strife when it's battling?
Answer: EMPOLEON
Explanation: Pearl Pokédex entry: 'It swims as fast as a jet boat. The edges of its wings are sharp and can slice apart drifting ice.'
Answer: UNOWN
Explanation: Silver Pokédex entry: 'Its flat, thin body is always stuck on walls. Its shape appears to have some meaning.'
Answer: MONFERNO
Explanation: Pearl Pokédex entry: 'It uses ceilings and walls to launch aerial attacks. Its fiery tail is but one weapon.'
Answer: FLYGON
Explanation: D/P Pokédex entry: 'It whips up sandstorms with powerful flaps of its wings. It is known as "The Desert Spirit."' Flygon isn't much of a spirit, though. o.o
Answer: MILOTIC
Explanation: Ruby Pokédex entry: 'Milotic is said to be the most beautiful of all the Pokémon. It has the power to becalm such emotions as anger and hostility to quell bitter feuding.'
Answer: LUMINEON
Explanation: Diamond Pokédex entry: 'It lives on the deep-sea floor. It attracts prey by flashing the patterns on its four tail fins.' Lumineon's actual tail is only one fin, so I suppose this is actually referring to the four wing-like fins along the middle of its body.
Answer: ARBOK
Explanation: Yellow Pokédex entry: 'The frightening patterns on its belly have been studied. Six variations have been confirmed.' However, we've only seen two distinct Arbok patterns in the games so far: one which is used in R/B/Y/FR/LG/D/P, and another one which is used in G/S/C/R/S/E.
Answer: NIDORAN
Explanation: R/S Pokédex entry for Nidoran male: 'The male Nidoran has developed muscles for moving its ears. Thanks to them, the ears can be freely moved in any direction. Even the slightest sound does not escape this Pokémon's notice.' However, there is no equivalent statement in any of the Pokédex entries for the female Nidoran.
Answer: MARILL
Explanation: Pearl Pokédex entry: 'The tip of its tail is filled with oil that is lighter than water, so it acts as a float.'
Answer: SHELLDER
Explanation: Emerald Pokédex entry: 'At night, it burrows a hole in the sea-floor with its broad tongue to make a place to sleep. While asleep, it closes its shell, but leaves its tongue hanging out.'
Answer: METANG
Explanation: Ruby Pokédex entry: 'When two Beldum fuse together, Metang is formed. The brains of the Beldum are joined by a magnetic nervous system. By linking its brains magnetically, this Pokémon generates strong psychokinetic power.'
Answer: HOUNDOUR
Explanation: Gold Pokédex entry: 'It uses different kinds of cries for communicating with others if its kind and for pursuing its prey.'
Answer: MUDKIP
Explanation: Ruby Pokédex entry: 'The fin on Mudkip's head acts as highly sensitive radar. Using this fin to sense movements of water and air, this Pokémon can determine what is taking place around it without using its eyes.'
Answer: BALTOY
Explanation: FR/LG Pokédex entry: 'It was discovered in ancient ruins. While moving, it constantly spins. It stands on one foot even while asleep.'
Answer: PIDGEOTTO
Explanation: FireRed Pokédex entry: 'The claws on its feet are well developed. It can carry prey such as an Exeggcute to its nest over 60 miles away.'
Answer: CATERPIE
Explanation: Yellow Pokédex entry: 'If you touch the feeler on top of its head, it will release a horrible stink to protect itself.'
Answer: KANGASKHAN
Explanation: FireRed Pokédex entry: 'The female raises its offspring in a pouch on its belly. It is skilled at attacking using Comet Punch.' 'The female' generally implies there is such a thing as a male too, doesn't it?
Answer: KOFFING
Explanation: Crystal Pokédex entry: 'If one gets close enough to it when it expels poisonous gas, the gas swirling inside it can be seen.'
Answer: UXIE
Explanation: Pearl Pokédex entry: 'It is said that its emergence gave humans the intelligence to improve their quality of life.'
Answer: JUMPLUFF
Explanation: Silver Pokédex entry: 'Drifts on seasonal winds and spreads its cotton-like spores all over the world to make more offspring.' So clearly, it happens by spores in Jumpluff's case.
Answer: PRIMEAPE
Explanation: Yellow Pokédex entry: 'It stops being angry only when nobody else is around. To view this moment is very difficult.' This is basically the same principle: if nobody else is around, there isn't anybody to view it. It appears that everybody thought this was either Parasect or Prinplup because their entries actually mention trees. Don't take my clues so literally, folks.
Answer: STARMIE
Explanation: FireRed Pokédex entry: 'This Pokémon has a geometric body. Because of its body, the locals suspect that is an alien creature.' So... Voltorb's body isn't geometric?
Answer: MUNCHLAX
Explanation: Pearl Pokédex entry: 'It hides food under its long body hair. However, it forgets it has hidden the food.' So... how long does it forget the food there?
Answer: LUNATONE
Explanation: Emerald Pokédex entry: 'It becomes very active on the night of a full moon. This Pokémon was first discovered 40 years ago at the site of a meteor strike.'
Answer: NOCTOWL
Explanation: R/S Pokédex entry: 'Noctowl never fails at catching prey in darkness. This Pokémon owes its success to its superior vision that allows it to see in minimal light, and to its soft, supple wings that make no sound in flight.'
Answer: ODDISH
Explanation: FireRed Pokédex entry: 'Its scientific name is "Oddium Wanderus." At night, it is said to walk nearly 1,000 feet on its two roots.'