A solution to the OP's problem is presented in the Hero book under the Teotl 'nagual' creature birthright - you can either have a nagual as a spirit animal OR transform into it.

This means that the ability to transform into one creature is a birthright that can be bought with XP or at character gen, the cost dependent on the number of dots in the creature - a standard tiger is likely to be a 2-dot beast.

There is, however, no guidance on how you would play this (do you switch out your character sheet for the beast stat block while transformed, or are you presumed to have...