Actually, you can use events to do this, very simple in fact.
First, you need to decide which key to use.
The keys that I suggest would be:
Shift or Z (A)
A (X)
S (Y)
D (Z)
Q (L)
W (R)
Any of these will work, just decide which one will best suite this.
you need to make two switches, one for activation, and another to switch between animal and human.
Go to common events, and make a new one, call it whatever, and make it so it is a conditional branch, then, on the last tab, choose button, select the respective one you want (The ones I put in parenthesis)
No, we simply make another conditional branch inside of that one, and that is if the switch for switching between animal and human is ON
ON will be animal, and OFF will be human.
So, if it is ON, then simply make the action under this conditional branch be change actor graphic, select the sprites for the character. and then a event that turns this switch OFF
Under ELSE you want to change actor graphic, make it be the animal sprite, and then turn the switch ON
under trigger on the common event you want it to be parallel and the switch that allows wether or not you have that ability.
now, once that switch is on, it will allow you to push the button of your choice to transform your character.
Ok, so that is the basics.
You can expand that to be much more complex.
I hope this made you understand what they were talking about.