Describes an action or event that happens abruptly, without warning.
He suddenly stood up.
by chance; accidentally; casually
Used when something happens without planning or intent, often with a light, casual feel.
I found the book I was looking for in a shop I casually dropped into.
Describes a quick, effortless movement, often implying a light jump or deft motion.
The cat lightly jumped onto the fence.
He suddenly popped his head in.
I casually picked up a pebble by the road.
The cat nimbly leapt over the wall.
突然 is a more formal, neutral word for 'suddenly', while ひょいと has a lighter, more casual nuance often tied to a quick physical motion.
ひょいと describes a quick, light, unthinking action (like picking something up or moving), while ぽいと focuses specifically on a careless tossing motion.
ひょっと looks and sounds similar to ひょいと but means 'by any chance' (as in ひょっとして), not 'suddenly' or 'nimbly'.
Mimetic word; the exact origin is uncertain, but it likely derives from the sound-symbolic root ひょい, which suggests a quick, light, or sudden movement.