Edit: I've incorporated some of the suggestions from the comments
I'm not sure how possible this would be in real life, but as you've not used science-based or hard science tags I think this should be believable enough.
What you're looking for is a Non-Newtonian Fluid. The most famous of these is probably corn starch in water (or custard, which often amounts to the same thing) which is a liquid but if enough pressure is applied to it it becomes solid and can be walked on, if you do it right. Now that's not quite what you are looking for, you want your unwary prey not to realize they are in a 'swamp' of sorts so needing to walk in a specific way is out of the question.
So here's where we start to play with things. We make the Non-Newtonian Fluid (let's call them NNFs for short) a lot thicker, so that instead of requiring careful high pressure movements to make it solid instead it's solid most of the time but if you move slowly and gently enough through it behaves like a liquid.
Edit: Apparently this is possible even without any handwaving, what we need is called a shear thinning fluid - the faster you try to move it, the easier it is to do so. Thanks to Agent_L and Aliden in the comments
According to this website it is possible to swim in an NNF, and in fact it's easier the bigger you are. That suits us fine as you want a fairly large creature.
So lets look at the creature itself now. It needs to be making slow and gentle movements to maintain the liquidity of the NNF so it's not likely to be a chasing predator. Ambush seems much more likely and sensible. So we want a creature that waits under the surface for it's prey to walk over head before springing up. As pointed out by anaximander in the comments springing should work fine, even if it causes the NNF to solidify a large and strong enough creature should have no problem bursting through a thin layer of solid ground.
Waiting for prey to walk over head isn't the best strategy though, you want to lure your prey over. Conveniently your creature also probably needs some kind of snorkel like device to breathe (I'm not sure gills would be effective in an NNF) and we can combine that with a luring mechanism, something that rises up and sits above the ground (in fact maybe it grabs the prey and drags them down, which might be easier than springing up through the NNF).
The first thing that comes to mind is an Angler Fish, one of these beasties:
Other options suggested in the comments include;
A snapping turtle like creature (suggested by Dent7777) - In real life snapping turtles already use a lure to hunt. Our creature would be a vastly scaled up version whose head was heavily camouflaged and disguised with a lure that sat inside it's open mouth. The head would sit above ground and wait for prey to enter the mouth before swallowing it.
A creature whose large mouth rises up to envelop it's prey and swallow them whole (as suggested by Andrew Cheong). I would imagine in this instance the creature would probably be snake like (indeed a Dune style sand worm, as mentioned in other answers, would fit the bill).