Kroq-gar is pretty cool fluff wise but he is essentially a Carnosaur-riding Oldblood which is hugely more expensive. Their stat line are almost identical.
He does get some cool magic items though, and an army trying to fit in more cold one cavalry would probably benefit from the core choice option. He would be interesting to have and he is probably one of the special characters I would take if I were to put one in my list.
Our special characters just aren't as powerful as some of the other races and aren't worth taking in most situations. In fact, they are almost the same as our regular characters but they have some sort of twist, aka Mazmundi on his Stegadon and Tenehuin with Lore of Beasts. I don't like taking special characters, our regular ones are powerful enough as they are.