What animals do we typically inject hormones in to that we consume? And what is the purpose of injecting them?

I believe Beef animals often have some hormones injected, Hogs typically have ractopomine injected (it's a beta-agonist, sort of an antibiotic). The purpose is to promote growth. A small share (and declining amount) of dairy cows have BST injected (bovine somatropin - a hormone that promotes milk production). This is declining because of consumer pushback against the practice. 

In all cases, there is no evidence that these treatments have any impact on food safety or on humans who consume the products. Humans don't have receptors for cattle hormones.