Pyramidion: Serenity. Go watch Firefly.
Junker: Rack City. I could say it's because it has two levels of gun racks, but it's mostly that I was listening to Tyga's "Rack City" when flying it.
Galleon: Womp Womp. It's the sound I generally make when it dies. It's so big, when you watch it explode the only thing you can really say is "womp, womp."
Goldfish: Nemo. It's the first name that came to mind when I thought of a fish. Yeah, I'm uncreative.
Squid: C.R.E.A.M. DMaximus, Plasmarobo, and I thoroughly enjoy rapping along to Wu-Tang Clan.
http://www.youtube.com/watch?v=bjZRAvsZf1g Similar to the junker's reasoning.
Spire: Power Tower. I thought it sounded clever. Afterwards I realized the spire achievement is actually called Power Tower, so now I feel less clever.
Mobula: Pancake. It looks like a pancake.