Night pigsy and Leafis rarely are underground, namely the later. And if we had the name of hte quest perhaps we could be of more assistance...
So, without that piece of the 'puzzle', I'm assuming that it's safe for you to go to the surface biomes that tend to spawn these during the night, like any Jungle or Forest. ;)

@Basan thank u for the info! I feel so dumb now x.x The quest said that the night pigsy spawned in fossil layer but actually you are right. They do spawn on the surface. I've already finished the quest and never did I step foot underground :)

This was the quest interface:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1550011975

Not a problem. Glad that was able to help out a fellow creator. ^^
Tbh, I recall that quest well because it's still bugged upon my Codex. Although I've already completed the one called "Ohhh, shinny!" (requiring me to hunt down all sort so Keepas), which if far ahead of that one at some point in the past while doing the that quest it started telling me that I needed to do that Night Hoglets and runt Night Leafies all over again and even after completing it again it still doesn't go away from my Codex. ;'p

You may have lost hte chance to build sucha spawner but, imho, those aren't really worth the effort... and that's mostly due to my PoV upon the resources spent/profit ratio (i.e.: blocks, time, naggy creatures while building vs. usefulness, constant usage factor). XD