LATEST IMAGE:

Ok, I've been itching to throw myself into a contest for awhile, and it looks like I'm gonna have some spare time in the next few weeks. So, lets do this!
Some of the early thumbnails and reference:

The direction I'm thinking of after playing with shapes in zbrush and sketching a bit:

So, crazy-fast running (and with those claws, climbing!) lizard with acidic saliva that'll melt metal to slag is the gameplan.
The idea for the rider right now is a wild-child using spears and "water" balloons to melt/rip through tanks, because what else would a kid with an acid spitting monster of rage do on the weekends?
Next thing after roughing out the two characters beyond blob-form in zbrush is exploring how something like an acid hose would look... the spears give it a cool silhouette, but I'm not sure if they're threatening enough.
Replies
I agree about the back leg proportions looking wimpy there, while fleshing them out further I made them a little bulkier. Though, I do want them to be lean rather than chunky. Hopefully this reads a bit better.
Next up I need to start on the rider, and add scales/details to the beast.