I assume you meant '사랑둥이' which means 'a person especially a child who is lovely or is loved'. Many Korean words are contracted and spelled differently for being sounded fun and cute. '사랑듕' should be contracted from '사랑둥이'. And I've never heard of 사룽댕 but it should be another word of '사랑둥이'. Here are more compound words of '둥이'. e.g. 쌍둥이(twins), 귀염둥이(cutie) Hope it helps and please follow me at koreanclass.canada on Instagram!

I'm just adding my comment here, but that's definitely the type of jargon used by women only I guess, because I've never seen the word before until this post lol.