Legendary Beast level requirements?

So I was just wondering.

As there a lot of different types of Legendary Beast species ; (ursa, devourer, crab, snake) and so on

Do some of these beasts require a certain area level before they start appearing?
I'm seeing more than others, and have currently not seen any Farric Taurus in acts 1 and 2.

Was just wondering if they CAN actually appear as they are a type of bull which isn't encountered until later on in the game.
I thought Legendary beasts ignored the local mob table in what spawns in certain areas.