I can name a so many times..especially Gabite in Sun and Ultra Sun It literally took me months to find it but in about four/five different runs..combined,In one of those runs I actually spent two days on it and got nothing...but in last run I actually found one..also finding Pidgey in the beginning of X took me about one hour (the last two playthroughs were like it)or so..and I even looked at the clock..but I guess it could be night too but I'm not sure..also Skitty was like two percent in Sapphire but sometimes it shorter/longer than others.

Edit:I forgot about Feebas in Diamond/Pearl and Platinum as well as the original Ruby/Sapphire it was so hard finding each time-however I actually gave up on it multiple times in Diamond and Pearl..because I kept trying but it never appeared once-not once >.> I've even tried looking online and copying exactly but still no Feebas.. I would like saying persevering it out and then actually finding that Pokemon helps the game but I'm not too sure..It's a pain to do each time through..for me anyway..

The most recent event was just the other day when I was looking for Zorua in Ultra Sun. I encountered every other pokémon on the route multiple times (including the ones that have lower encounter rate than Zorua) for almost 20 minutes before I finally found Zorua. Due to my first attempt to catch him ending up in failure, I had to search again. In total, it took me over an hour to find and catch him.

Man, I STILL can't find Rockruff in Ultra Moon, and it's at the beginning of the freakin' game. *Siiigh*

Two words: Female Salandit