This video contains two beta creatures: a Temple of Time/Palace of Twilight enemy and a Goron Golem!
They only appear in the game by using certain Action Replay Codes...

Thanks to JaytheHam, brkirch, Twilight Spectre and Y.S. for hacking such amazing codes!!!

Because of the many requests, here are the Action Replay Codes:

(m)*
PAL:
NHYH-GJQ6-03Z1Y
R8M1-7MZM-3R7AW
NTSC:
BJ1W-52P2-KZQZT
KNY1-WU69-VUVMY
*This is the so called Mastercode, which have to be added when adding a new game to the Action Replay. These (m)-codes differ from the ones codejunkies provided. Better use the ones I posted. The PAL-(m)-code was hacked by JayTheHam, the NTSC one by Twilight Spectre.

Epona is ToT creature (Press R) Code by brkirch
(PAL)
5QBQ-MZ0M-H6R60
6BY4-0P8A-FQ5JC
RTVC-Y466-M1CZ0

(NTSC)
1HUE-83V7-6T801
AUKR-HM85-VKQXC
HRAF-7TPU-KWHYQ

Epona is Goron Golem (Press R) Code by brkirch
(PAL)
957R-NC71-P4YGG
6BY4-0P8A-FQ5JC
3431-ABXC-4PFJU

(NTSC)
ZVKQ-NDF8-2PBQR
AUKR-HM85-VKQXC
NRW5-5BZB-AKH6Y

USE ONLY 1 CODE AT ONCE!!!

Note: To activate a code in the game, press the R button once and go to another area. After that, the chosen monster will be everywhere, where Epona would have access.

Have fun! (less info)