Heroes V S.T.A.L.K.E.R

I installed a mod that replaces the regular centaur with a gnoll, and SUDDENLY it turned out to be a ranged attacker.

It's simple: because the mod author added the ranged attack ability to gnolls (which they are, if I'm not mistaken, in Heroes 7). If this ability hadn't been specified in the creature's files, they would have remained melee units. So, the "ranged attack" ability of gnolls is not because they are replacing centaurs, but because the mod author gave them that ability (he could even have given them a lightning strike ability ). I also assume that Hottabych's chimeras will not be ranged units. But these creatures certainly sound intriguing, and I would be happy to see them in action.