The LFS we go to sells red-clawed crabs. These do fine in fresh water but aren't really "fully aquatic." I got one for one of my tanks, and I keep the water level just low enough so that there's an island formed by a piece of driftwood. He spends a decent amount of time out of the water. He mostly eats whatever he can get has claws on (usually algae wafers).
He only lives with fast moving fish, but I've definitely seen him try to reach out and snag a passing barb. I definitely wouldn't recommend him with a betta, as their slower speed and huge trailing fins are basically an invitation to get jumped by a crab. They're fun critters for other types of tanks, though.