The cloaca holds both the penis and the butthole on a male, and the vagina on a female. They still have penetrative sex. They're not fish. It just doesn't poke outside the body.
Can you back that up? I've spent the last 10 minutes searching up cloaca diagrams and pictures and articles and I can't find any decent information about it. Only saying that they do a cloaca kiss and transfer sperm, but then I can't find a cloaca diagram that labels any part as a penis.
I couldn't find anything specific to peacocks either, but plenty of various other birds, including chickens which have the smallest little nub of a penis to ducks and their long, twisty corkscrew cock.
Yeah, I'm not sure you could call whatever a chicken does "penetrative", and I feel like the term "penis" has a specific meaning that wouldn't include cloaca.
cock (n.1)
"male of the domestic fowl," from Old English cocc "male bird," Old French coc (12c., Modern French coq), Old Norse kokkr, all of echoic origin. Compare Albanian kokosh "cock," Greek kikkos, Sanskrit kukkuta, Malay kukuk.
cock (n.3)
"penis," 1610s, but certainly older and suggested in word-play from at least 15c.; also compare pillicock "penis," attested from early 14c.
They're called peacocks because they're peafowl who are cocks. It's a way older term than the slang usage.