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Well, the main attributes are Size and Diet, with size requiring a minimum to allow for ingestion of a person, and diet requiring the Pokemon to be interested in doing so.
Of course, if you treat Pokemon like real organisms, they are not going to be swallowing anything the size of a human whole unless they are massive, which there are only a few that could reach such a size, but then the issue becomes why would a creature the size of a building bother with food as small as a Human unless it is planning on eating hundreds of thousands of them?
Realistically, I could see snake-like Pokemon like Seviper, Serperior, Arbok, Onix, and Steelix being candidates. While not snakes, Gyarados is large enough to pull it off, as well as Kyogre and Wailord, but Kyogre's ecology is completely unknown and Wailord likely eats small organisms like its real-life counterpart. Lastly, I'd like to mention Huntail and Lantern as possible candidates due to their resemblance to deep-sea predators, which are often capable of consuming prey much larger than would be expected relative to body size.
There are many pokemon that could eat humans, but very few that could actually swallow them whole unless they were exceedingly larger than canonically shown or they were consuming juvenile humans.
Honorable mentions to Muk, Gastly, Haunter, Ditto, Swalot and Garbodor for being amorphous and able to envelop a human being. Jellicent could theoretically snare a human as well, judging by their pokedex entries. Carnivine and Victreebel have canonically swallowed people before, but neither are capable of actually "swallowing", just keeping a human inside of them. There may be other pokemon that could do it, but those are the ones that I could think of with real-world analogs or physiology that could permit it.
