Zoids: Chaotic Century

The premise behind the toys is simple enough - assemble a massive array of numbered parts around a custom engine to form a mechanical animal, then wind a knob or flip a switch, and presto, the entire assembly comes to life, walking and moving. The anime helps to reinforce the mythos written around these machines - they are naturally-occurring lifeforms on the distant planet of Zi, but harvested and adapted into weapons of war by human colonies, divided into the Federal Republic and the Guylos Empire. At the time of the cartoon's debut, the last great war has long ended and both sides have entered a state of ceasefire, but recent events involving the possible existence of insanely powerful Zoids threaten the peace.

The hero of the show is Van Freuheit, a young boy and son of a deceased war hero, who in one day stumbles upon two new companions who will change his life forever - one is Zeke, an Organoid or naturally-occurring Zoid which resembles a tyrannosaurus rex, and Fiona, who is later revealed to be the last survivor of what one could call the Native Zi people. Their adventures take them out of their peaceful village in neutral territory to the very top of Republic and Empire rule, while meeting and befiending unlikely allies like mercenary Irvine and parts dealer Moonbay, and thus change the history of Zi for the better, as a mutual alliance is established just in time for the show's second season, Zoids: Guardian Force.

Trivia notes:

Two types of Zoid will reappear again in future seasons of the show - one is the generic Liger, which appears first as the Shield Liger, evolving into a Blade Liger, and finally being completely redesigned into the Liger Zero from the third season onwards. The other is the Command Wolf, which apart from color changes and different weapon systems, looks exactly the same in every season.

(Sources: 'Zoids: Chaotic Century', 'Zoids: Guardian Force' and 'Zoids/Zero' cartoons)