Without beating around the bush: Monoceros (it means it has one horn. Ha ha. ), from Grand Kingdom.

Seriously, with how powerful this thing is, you'd expect it to have wings too! It can be a real pig to deal with f you don't know the method for beating it or your party is at a high level.

Since i apparently can't find any images of it that aren't on Yotube for... reasons, i'll just outline the fight here. Consider this a free tip while i'm standing on a soapbox (whoever brought that in, thank you ).

Okay, so for one thing, even if its attacks are incredibly powerful, it telegraphs them quite clearly.
1: Creation. Monoceros will flash green and raise its head, charging this spell. When it next comes to its turn, it will create three large green crystals at randon locations on the field. While these are present, Monoceros is invulnerable.

2: Spark Ball. After that, it'll lift it's head again with a spark briefly appearing. When this spell is cast, it'll destroy one of the crystals, shocking any in the area around it for massive damage.

3: White Blitz. After the third crystal is destroyed (It is vulnerable during this time) it'll lower its head as if taking a running stance. When THAT is charged, it'll teleport across the field, leaving lightning in its wake. This is a full-field move and good luck avoiding damage from THIS. After that, usually it'll charge Creation again. Unless....

4: Holy Sword. The beast will go dark and its horn will glow red. When this goes off, the screen will flash black and anyone close to it will take a ton of damage.

So... yeah. I hate this thing. I'm so glad i got this off my chest.
You can have your soapbox back now.