Disclaimer: I am not J. K. Rowling. I do not own Harry Potter.
Note: The following is set in a universe where the direction of conversation towards the end of the 'detention in the forest' episode of the Philosopher's Stone year, took a slightly different turn, and Hagrid's friend Aragog got mentioned.
Hogwarts, 27th May, 1992
Dear Father,
Just to let you know that the Hogwarts groundskeeper may not be such a bad sort after all. We ran into some centaurs in the Forbidden Forest last night and they gave him a darned good kicking. They seemed to think that he tried to wipe them out by releasing acromantulas into the forest several years back. The headmaster arrived only just in time to pull them off him. I will try to discover if Hagrid has implemented any other anti-centaur plans over the years.
Your dutiful son,
Draco Lucius Malfoy.
PS
Would you ask mother to send some more cauldron cakes, please?
Author Notes:
Harry Potter wiki (at the time of my writing this piece) fixes the date of the detention in the Forbidden Forest where Harry & company run into the unicorn-attacking horror and the centaurs as being the 26th of May, 1992.
Unless the centaurs of the Forbidden Forest have some sort of alliance with the acromantulas (which in the absence, to my current knowledge, of any mention of in canon, I imagine to be highly unlikely) it seems to me that the centaurs would view the acromantulas as pests, if not outright dangerous threats. Unless the centaurs are herbivores, they will likely be in competition for some food with the acromantulas, when it comes to hunting, and irrespective of what the centaurs eat, it seems likely to me that an acromantula (being carnivorous) would be quite prepared to eat any centaurs it could catch.
This short piece assumes that the centaurs are very unhappy with the Forbidden Forest's acromantula colony, and would like to slowly and horribly kill whatever idiot witch or wizard released said giant flesh-eating spiders into the forest in the first place (not only bringing in first a male, but then getting a female in too, so that the wretched things could breed and multiply). It's just that the centaurs never found out until the previous night who it was...
I've assumed that a young (and relatively impressionable) first-year Draco would look down on centaurs - even more so than on Hagrid - and would actually be quite impressed by what the centaurs apparently considered an attempt to wipe them out.
This story is a one-shot.