No this hasn't been proposed before (or I don't remember), but I was thinking about it - not only for creatures - and I'd like to do it.
The main problem is that before starting to work on it I'd want to be sure it 1. will get artists' support (I guess no problem with this), 2. will look good/won't hurt gameplay/will be accepted by players. It looks ok on bare ground in your examples, but can you make an example image with a real map/other objects around?
Sorry for the delayed response.
pic. 1 - elephant 3x2, yak 2x2, troll 2x2, hydra 3x3.
pic. 2 - cyclops 3x3, ettin 3x3, hippo 3x2, demon 3x3, demon 2x2.
pic. 3 - yak 2x2, hippo 3x2, deer 2x2, demon 3x3.
pic. 4 - elephant 3x2, yak 2x2, hippo 3x2.
In general, the most illustrative example in a narrow corridor - creatures 2x2, 3x2 and not shown 1x2, in my opinion, fit perfectly. The main thing when drawing not occupy all tiles offered.
3x3 certainly does not look very well, especially the demon 3x3, which is stretched to the maximum possible area - 300% sized sprite. But hydra is 200% sized sprite, placed in center 3x3 area looks tolerant.
In pic. 4 can see that 2x2 not look very much(but not for me) in a vertical narrow corridor, but this is solved by drawing the sprite as far as possible to the left or transition to 3x2, where can draw sprite symmetrically.
Also, remember/seen this? http://www.bay12forums.com/smf/index.php?topic=138754.msg7390751#msg7390751
Saw many times, but only after you pointed out, I noticed that there is a multi-tile furniture)