Various characters refer to Richard as "the boar" throughout the play. What's up with that? Well, Richard's heraldic symbol (on his coat of arms) is the white boar, which is often considered to be a fierce and hideous creature...just like Richard. (Check out his coat of arms here.) So we're not surprised when Lord Stanley has a dream that a "boar had razed off his helm[et]" (3.2.5) – it doesn't take Sigmund Freud to figure out that Stanley's afraid Richard is going to cut off his head.
Earlier in the play, Queen Margaret declares that Richard is an "elvish-mark'd, abortive, rooting hog," which is a not-so-nice reference to Richard's emblem and the fact that Richard (according to Shakespeare's play) was born prematurely and "deformed" (1.3.11). Margaret's not the only one who suggests that Richard is more animal than human.
Later, Richmond calls Richard a "foul swine" and refers to him as a destructive boar during his speech at Bosworth field:
The wretched, bloody, and usurping boar,
That spoil'd your summer fields and fruitful vines,
Swills your warm blood like wash, and makes his trough
In your embowell'd bosoms, (5.2.1)
Translation: Just as a boar might destroy a field of crops or feast on a man's guts (ever seen the famous "killer boar scene" in Hannibal?), Richard has ruined England by taking the throne and trampling all over the rights of his people.