There's probably a lot, and depending on the CR, may be more or less devastating depending on what appropriate ECL PCs might have to deal with.
Taotieh (CR 11) is a Large marble cat that can swallow up to four Large-sized creatures and they're put into a lightless pocket dimension (so likely no worry about bags of holding or portable holes). They take no damage, but there's only 3 rounds of air. So they're in the dark and basically have about 3 + Con rounds to escape (assuming they're taking actions, like trying to cut their way out). Plus when a creature 'cuts' their way out, it doesn't really hurt the taotieh, they seem to leap from its mouth. It doesn't cut a hole or antything, so others inside can't also get out and it doesn't prevent it from using swallow whole again (most creatures can't use it again until the damage is healed) and the prison interior has hardness 8.
Double-headed Snake CR 15) is a Gargantuan anaconda with two heads, so it has two bite attacks, both of which can grab a creature, and it has fast swallow, so any grabbing creatures can be immediately swallowed as a free action at the end of its turn instead of waiting to start its turn with a creature held in its mouth. It deals immense bludgeoning and acid damage per round, and it has an intimidating gaze that can paralyze targets.
I can't tell if it has to use both heads to do it, or both heads can each do a free action gaze, I think the latter. Also, the paralysis ends if the double-headed snake attacks the target, but I don't know the ruling if the snake's tactic was to bite and grab, and then free action gaze (with the other head, since as a GM I'd rule the biting head can't really gaze at a creature in its mouth), and then paralyze and free action fast swallow. If the fast swallow is counted as an 'attack', then the swallowed creature might have several rounds of not being able to escape, but that's a GM's call.
Otherwise, you can check d20pfsrd and search for 'swallow whole' and click on 'bestiary'. That will give you a list of creatures with the ability, you can click through them for the CRs.
