In our early days of playing Monster Hunter and... hunting monsters... there was always that one monster near the start (or even in the middle) that you'd always struggle against or often get triple carted against, either due to lack of experience or not preparing for it well and setting off anyways... which you could consider as a lack of experience Anyways, for me, this monster was the Tetsucabra. Hoo boy. Back when I first started playing I thought this was one of the toughest monsters in the game. Pouncing everywhere, doing massive damage by crushing a boulder with it's jaws, and being deceptively nimble. I triple carted many times, but it was due to inexperience and not realizing that I can't just be trying to attack it willy nilly. Anyone else have any similar stories?
Plesioth. Plesioth gives me trouble even now...it spends forever in the water out of range, it's absolutely massive, and it has the weirdest hitboxes imaginable...that damned hip-check. Yian Garuga was actually pretty difficult too, if only because it was like a faster, smaller version of Rathian.
Black Gravios, in Freedom 2 this thing was my nemesis in every way you could imagine. I ended up learning a new weapon just so I could kill it because it liked staying in inaccessible areas and just do fire beam spam. So I learned bow just so I could even hit it and that thing have a ton of HP so it took forever to kill it.