Unfortunately the list of names doesn't include the actual models being used. Some of the speculation that they may be draekadon and void terror models may have arisen from the fact they've been on the untameable skins list as suggestions for, well, since the forums were on Mania's Arcania

Both are very obviously derived from the core hound base model, and neither is that different from a standard red core hound compared to the differences between a red core hound and the white fluffy one (which are huge). But I always had them on the list as "may not happen but wouldn't it be nice if..?" suggestions.
If I had to guess from the names supplied, I'd suggest that "Portent" would be another fluffy model (Portent being a synonym for Omen). It'd be nice if he was a different colour, but there's been no sign of a new fluffy skin on the databases yet. Magmagar and Felmaw would be red and green standard core hounds respectively, and Chrominius would be a draekadon model, related to Chromaggus in Blackwing Lair. Of the two suggested models, the drakeadon makes the most sense to use. In Blackwing Lair, Nefarian was performing "genetic" experiments to create the chromatic dragonflight. I don't know if Chromaggus's origin is ever explained, but it could be assumed that he was the result of an experiment in infusing the chromatic essence into one of the core hounds that live close by.