Strength is a creature attribute in Black & White. It determines which objects a creature is able to carry as well as how much damage it can deal with attacks in combat. It ranges from 0% (weakest) to 100% (strongest).
Strength varies from creature to creature, with felines generally being the strongest and primates being the weakest (although exceptions like the Gorilla do exist).
The strenghten creature miracle increases this stat to 100% no matter how low it is, although it is only temporary.
Lack of exercise, and constant sleeping can decrease a creature's strength, gradually reducing it back to 0%.
The weaken creature miracle decreases this stat to 0% no matter how high it is, although it is only temporary.
The strength of a creature can be seen at any time by going to the creature cave and checking the scrolls on the wall there (pressing F5 will instantly take the player there).