That animals can vary in physical capacity such as strength and agility due to training, genetics, or conditioning is a matter of everyday observation and biological fact, requiring no citation.
The claim states that animals within the same species can become noticeably stronger, faster, or more agile through training or individual differences. This is a basic, observable biological reality across the animal kingdom. Per the instructions, trivially true observations do not require academic citations and are classified as COMMON_KNOWLEDGE.