That may be true... I didn't play storm of magic so it entirely possible. But without any expansions and using only the core rules, they are an extremely sub optimal unit. Monster mounts are typically a poor choice because they can be shot out so easily from under your character. In the manticore's case, this is especially problematic since he is so squishy. The best mount options for WoC is either the daemonic mount (for immunity to killing blow) or my favourite the disc of Tzeentch for mobility. In either case, field the unkillable "Hortennse" Chaos Lord/Hero.
And thus field the ridiculously OP Lord Choice that can kill an entire army by himself and break the game... That's why Dwarfs now get to roll two artillery dice instead of 1 for the Organ Gun...
He is definitely epic. I like to jokingly think that he possesses the special rule: immune to warhammer. Of course, he can't kill an entire army single highhandedly (his offense while good, isn't exceptional), but he is definitely the Swiss Army Knife of the army. When I field him, I use him to kill or neutralize my opponent's greatest threat. He won't kill a death star by himself, but he can bog it down for the game. He is the perfect counter to just about anything overpowered that my opponent may field. As I tend to be a defensive player, he is the ultimate build for me. I realistically only fear a couple of things with him: anything that will disable or destroy magic items (as my build relies on his ward save and crown of command stubbornness) killing blow as my lord is mounted on a disc of Tzeentch and is still "susceptible" to it (though the ward save still boasts a slightly better than 3/4 of a chance of nullifying a successful incoming killing blow strike) A character such as Hellebron will also stand toe to toe with Hortennse Chaos Lord, though I can tweak him to come into the contest as the odds on favourite to win. Edit: I should also add that I use this Chaos build to counter the old "no predatory fighter attacks from supporting ranks" debate. If I feel someone is trying to cheat me out of that rule (which is always best discussed beforehand), my Lizardmen army is upgraded to my WoC army featuring 2-3 of my best chaos lords/heroes.