Bottle

The black cap has a windshield washer symbol right? It is windshield washer fluid reservoir.

Add Windshield Washer Fluid Toyota Prius (2004-2009) - 2005 Toyota Prius 1.5L 4 Cyl.

 

That white plastic can and all the weird bent black hoses in the pictures are part of the throttle body air intake system. Not sure if its for sound deadening or part of the atkisson cycle mode weirdness where the intake valve closing is delayed in the compression stroke thereby back feeding some gases.

That back feeding is quenched by the big plenum tank in the bottom of the intake manifold. There's a big bulbous tank under the throttle body that absorbs the pushed back gases from the compression stroke.

In picture 2714 the big black intake hose with the bellows connects to the white can under it.
Its like some sort of holding tank for intake air. I suspect its to quiet down the intake air.
Lots of cars have intake anti resonator.

 

The words for today are "Helmholtz resonator".

In addition to quieting the intake, it reduces the impedance of the intake ducting, at chosen RPMs. It is a good way to win bets with people who sell mods to "reduce intake restrictions", some of whom may not have taken Waves.

 

Or if you are talking lower, just behind the bumper, it would be the thermos for the coolant.

 

A picture of it would be helpful.

 

I found this pic of the thermos. I was thinking it was on the passenger side, but not according to this pic.

 

Thermos is on the driver's side (maybe different, in right-hand-drive countries).

No room for it on the passenger side, 'cause the Helmholtz resonator is there.