Hum... as to the animals: don't you think dinossaurs is a bit too obvious? There are LOADS of other alien creatures capable of disassembling and eating a mech... Don't you guys have a TV at home? ;)
You can include giant worms... but they're stuck to the ground; when you walk over one you fall, it eats you and uses its new food to create 10 more like itself and die :)
You can also include bugs with corrosive acid capable of digging trough plated armour...
And plants that when touched explode (like hydrogen trees)...
See, there's lots of creatures you can use, just let me think of some more :)
