These guys could be utterly fantastic!
Also mentioned are "Riverbeasts", which are based off Hippos. Here's what we have from the Petopia Forums - "Riverbeast (name confirmed by Slickrock, who has also seen them in the wild). We don't have any model shots yet, just this concept art. He confirmed that it is a lot like the concept art, but (quote) "Body a bit longer, not quite as thick, and face much more hippoish, but that is certainly it."
Additional news - we're gettin' Hydras as tamable beasts, which is sweet, though I wonder if they will be as loud as that ghostHydra a few people managed to get.