>>53782551Palkia - Theropod, Pearl, Space, "arkia" of Diarkhia (dual rule)
Dialga - Sauropod, Diamond, Time, "Diar" of Diarkhia
Giratina - Centipede/Ghost, Platinum, Antimatter, Fallen Angel
Palkia is water because pearls are found in clams underwater. Dialga is steel because diamond in Japanese is 金剛石 (kongoseki, "strong metal stone"). Giratina is ghost because it comes from another parallel universe. All three are Dragon because of their dinosaur-like features. Dinosaur in Japanese is 恐竜 (kyoryu, "scary dragon").