This example of an involuted thymus is from an elderly cat.
Involution is a normal process.
As an animal ages, the functional parenchyma of the organ is replaced with fat and connective tissue, and the organ as a whole diminishes in size and as a proportion of body weight. It does not become totally nonfunctional, however. Natural involution, as in this specimen, is an irreversible process. But accidental involution due to some exogenous agent, such as chemical or radiation insult, can usually be reversed once the offending stimulus is removed.