fishing trap
羀 centers on a device for catching fish: a fishing trap or weir. Its meaning is narrow and concrete, referring to the tool itself.
羀 combines 网 (net) with 畱, which likely contributes the sound. The character is a phono-semantic compound for a fishing trap made of netting.
The top 网 is a net, and the bottom 畱 looks like a complex trap structure. Picture a woven net trap set in a stream to catch fish.
For リュウ, imagine a fish trap shaped like a long, flowing river current: the current's 'ryuu' sound cues リュウ as the trap catches fish in the flow.