Disclaimer: I'm not muscling in on JK's turf - just gambolling on it, like a spring lamb, having fun working out the literary and psychological puzzles which she is having fun setting us.
MONSTER
"Oh for pity's sake - not another monster? You really cannot keep bringing these things into the school. Surely you realize that?"
"But I hatched the egg meself, an' she won' do no harm, I know she won' - look, she's only little, see, not much longer 'n me thumb. An' pretty!"
"I know you think that every be-tentacled monstrosity you meet is really just a great big softie that's been misunderstood... but sooner or later one of your little pets is going to kill somebody. I mean honestly - a basilisk! Really, you've gone too far this time, Salazar."
Author's note:
According to Fantastic Beasts and Where to Find Them, it was considered remarkable for Herpo the Foul's basilisk to have lived "close on nine hundred years", and the males have a scarlet crest. There's no mention of Slytherin's basilisk having a crest, and it was in his possession about a thousand years ago - so it's a female, and must have been very young when he had her.