Now now, hear me out.
We already have a system in place for doing diagonal, vertical, and horizontal objects - the vehicle system. It can translate a specific template for various directions and IMO does an excellent job of it. Why not steal this and make a similar system for creatures?
You could even go so far as to have differing tiles have different stats - one tile for example might be crucial to the creature being alive while others are superfluous- something like those creepy purple eyeball creatures or whatever they are that keep growing back, while only the center is vulnerable. Or something like the underground worms – only the head can actually die, while you could make a multi-tile body. Not only would it make many creatures a lot more interesting, it would give a much higher amount of combat depth and give obvious uses in flanking and rear attacks.
In addition, why not make the player able to grow to multi-tile size with the Huge line?
Creatures that could use being multi-tile:
Bears (2x2)
Moose (1x2)
Graboid/worm (1x5)
Triffid Queen (2x2)
Giant Frog (2x2)
Shoggoth (2x2)
Tripod (2x2)
Chickenbot (3x3)
Tankbot (4x4)
Fat Zombie (2x2)
Zombie Brute/Shocker brute (2x2)
Zombie Hulk (3x3)