Of the mobs people XP on there are a few pains, especially if people have never tanked them before or don't know how to tank them.
Coeurls - as mentioned above have annoying tp moves (silence and paralyze)
Cockatrices - annoying tp moves, and can be straight fatal if your tank directs it into the party or facing it for that matter.
Wyverns - lots of status effect moves (silence, paralyze and poison) plus they hit like freaking trucks, they have a three hit tp move that if it full connects will drop a ninja without shadows 100% of the time
Funguars - silence, paralyze, disease, and they attack fast (and more annoyingly link)
Corses - (people used to xp on these ... haven't seen it done on Nasomi to date, but in the era they did) .... SUPER annoying AoE silence, Curse, and Charm (ick).
As for others mentioned Anticans tend to be sort of pushovers, most people don't touch them, and silence doesn't have a huge range from them imo.
If you lack stuns, then goblins, bombs, and tonberries are super annoying. If you lack a ninja tank then triple bats, spiders, pugs can all suck as they all have single target high damage attacks.
Biggest pains though Fomors, Diseased, Sabotenders, Opo Opos, and Tonberries. Fortunately none of which appear to be common XP mobs. Tonberry hate SUCKS.
Forgot to add some historic XP mobs ...
Efts - AoE Dispel, AoE Stun (that ignored shadows), Poison that ignored shadows, and AoE tp moves
Raptors (oh Cape Terrigan) - Cone based Stun, Slow, and Paralyze ... when you tank in those tunnels or against wall tanks usually porly position it and aim it right into all the mages, so that's a pain.
Morbols .... I think even if not glitchy and casting instant aoe death (Extremely Bad Breath), they are annoying.