No, you don't need a regular Pokemon, to use it as an EX card. They're better and more rare.
Pokémon-ex are typically much stronger Pokémon than the Pokémon they are based on. They usually have significantly higher Hit Points and more powerful attacks. To counter this, a ruling was put in place saying that if one was defeated, the opposing player would be able to take 2 prize cards as opposed to one. As the Pokémon are called ____ ex, you could potentially have four of one Pokémon and four of that Pokémon in ex form in your deck.
Hope this helped!