Although this monster at the end of both Books 5 and 6 may not be the most interesting or fearsome in the whole of The Faerie Queene, we've got to give him points for having the coolest name ever.
The Blatant Beast, the monstrous agent of Envie and Detraction, is, just like them, associated with slander, lying, and general bad-mouthing; the Blatant Beast is a reimagining of slander in monster form. It chases poor Artegall after he's successfully defeated Grantorto as an enactment of the lies that are supposedly spread about Artegall's deeds.
Claridon then spends almost all of Book 6 trying to track down this monster, whose threat is less his brute violent force and more his pernicious sneakiness. However, as Serena and Timias can attest to, the Blatant Beast is not to be taken lightly; his wounds prove almost incurable since they attack not just the physical body but also the spiritual health within.
Spenser clearly felt some personal connection to the threat of the Blatant Beast since he thought that his poetry wasn't receiving the praise that it deserved. It's on this depressing note that the whole poem in fact ends, and the Blatant Beast, unlike any other monster in The Faerie Queene, kind of wins.
It turns out that Calidore's muzzle is only temporarily able to stop the Beast, and that he soon escapes to terrorize the world with a vengeance. So while the Blatant Beast in many ways appears to be one of the least scary monsters Spenser dreamed up, he does get the last laugh.