Wow, Shark's #3 is definitely heartbreaking. (The others in that post are too jocular for their own good.)
I must admit I have a soft spot for Bolt's Mittens — if you look closely at the animation, in that particular scene she's actually holding back tears.
I guess a crowner is not a bad idea, but I feel I should point out that the Lady and the Tramp images do not as clearly give off a prisonlike feel, given the wood, the green paint, (which looks like, if not actually is, plant material) and that the bottom 2 images have a fence instead of the bars.
The problem with this one is from a real pound and looks like a real pound. This fictional trope is quite different to real life and we don't want to evoke the later.
The Bolt pic seems to be a good visualization of the trope: it looks like it is emulating a human prison.
I'd suggest a lighter version of the Bolt pic so we can see the character more clearly: