Zekrom is a legendary Pokemon, so it isn't easy to raise. It is part of the Slow Experience group, higher than the slowest group that requires the most EXP to Evolve. That group is called the Fluctuating Experience Group.
The final of the four Generation I experience groups, the Slow group features the highest amount of experience required for a Pokémon to reach level 100 in Generations I and II, and the second highest amount since then. Containing many rare, powerful, and legendary Pokémon, Pokémon in this group are typically very hard to raise. At level 100, a Pokémon in this experience group will have 1,250,000 experience points.