The froghemoth has both lightning resistance and Shock Susceptibility; is this an error? Are there other monsters like this?

This actually is a reasonable result from a creature biology standpoint. The garter snake and rough-skinned newt are in an evolutionary arms race. The newt is toxic for creatures to touch or eat, but the garter snake can eat them safely. However, exposure to the toxin (eating too many newts) does cause the garter snake to slow down. See An evolutionary arms race: resistance vs toxicity.

This is pretty much exactly the same effect here. The froghemoth resists Lightning damage, but is slowed down in process. In essence, the body is using resources to resist the damage. The designers have decided to represent this as roughly the same effect as the Slow spell.

I think the intent is clear from how the two work together. The froghemoth takes half damage from lightning, but on that turn the froghemoth has a hit to its AC and speed. My guess at the author's intent is that doing all that other stuff and taking the full amount of damage would have made the monster too weak, so halving damage allowed them to add the interesting effects.

Moreover, in previous editions (3.5 at the very least), the froghemoth was immune to lightning damage, but would instead take a speed hit. As far as 5e goes, I don't recall any other creatures that have both a similar resistance plus other negative effects from the same damage type.