It has the power to control time. It appears in Sinnoh-region myths as an ancient deity.
– Pokemon Diamond Pokedex
Dialga is the mascot Pokemon for Pokemon Diamond, appearing on its boxart. It is the Temporal Pokemon, said to be able to control time, yet it can still be captured by a trainer (though according to the game, only a trainer pure of heart, aka you XD). It’s pretty big, being 5.4m (17’09”) tall, taller than Palkia. And probably due to it being part Steel, is twice as heavy as Palkia.
I’m not sure why Dialga is part Dragon. I think it’s part Steel due to connection with Diamonds, and Diamonds being hard like Steel. I think it’d make more sense for it to be part Psychic (as overused as that is, especially for legendaries) because it can control time.
Dialga’s typing is unique. It’s the only Dragon type not weak to Dragon type attacks. It’s also one of the few Dragon types not weak to Ice. It’s still weak to Ground and Fighting though.
In the anime, Dialga is featured in the 10th movie, Rise of Darkrai, where it battled Palkia. It was also seen in the 11th movie where it battled Giratina. In the 12th movie, it was also battling with Giratina and Palkia. It seems to be always battling, haha. Although it did help out the trainers in the 12th movie.
Stat-wise, since it’s legendary, it’s got 680 total stat. It has very high Sp Attack (the highest of all Steel types, and tied with highest Dragon type).