They sent Marines to check up on the colonists, not “specifically to fight the creatures.” They were well-armed (until they couldn’t use all those arms, deep in the terraforming plant), but not especially well-trained at all for the mission they eventually realized they were on.
And here it is (definitely NSFW): The Long Tomorrow, by Moebius | All That I Love
This earlier thread will also be of interest, I’m sure: Aliens - so what were the Colonial Marines actually prepared for (tactically)?