At the time, every one was saying how this would be a pretty sweet Commander, especially considering that it has Bestow on it! How interesting would it be to Bestow this from the Command Zone? To this day, I still wonder about it. But I'm sure that the Wizards of the Coast design team had their reasons.
Anyhow, Chromanticore, by itself, is a 4/4 with flying, first strike, vigilance, trample, and lifelink. If you Bestow it, which costs 2WUBRG (2 colorless, White, Blue, Black, Red, Green), the enchanted creature gains all of these things. It's very powerful. If you manage to get all five colors of mana available to you, it's going to be a solid finisher.
In Standard, it's understandable that Chromanticore didn't do very much. Being a five-color card in an environment where many decks were three colors and perhaps splashing a fourth didn't really let this card shine. There are ways to sneak it out in Modern and other formats, but there are such juicier targets to consider that why would you bother?
Besides the fact that Chromanticore is not a Legendary Creature, some players have decided to unofficially use it as a Commander. Basically, decks built around the Manticore have focused on running almost exclusively Enchantment Creatures and other powerful Enchantments. Many friendly playgroups already allow Chromanticore as a Commander since he's not overpowered. These are decks built purely for flavor, but they look fun. Still, if you're taking the deck to an unfamiliar playgroup, be sure to have a backup Commander on hand, such as Horde of Notions.
Officially, Chromanticore sees a fair amount of play for a five-color card in Commander. Interestingly enough, the Commander who uses the Bestow creature the most is Progenitus, which is odd since you can't attach anything to something with Protection From Everything. Of course, there are other creatures in the deck you can attach it to.
Chromanticore is pretty fun in Child of Alara decks. Attaching the Manticore to it makes for a 10/10 with flying, first strike, vigilance, and lifelink, as the Child already has trample. Karona, False God and Cromat decks also utilize the Chromanticore's talents sometimes.
Honestly, it's a bit strange that Chromanticore wasn't made a Legendary Creature in the first place. The flavor is there, and who realistically can play multiple copies on the board at once? You can see why this is an unofficial Commander for some people. It's a casual favorite, for sure. It's a well designed, powerful mythic rare that could have been quite a Commander (officially) if they'd just made it Legendary.
And 5-color Bestow never became a thing in Standard or Modern... But you never know!