I think one of the best additions to the current Slytherin house pets would be a horned serpent. Horned serpents can be befriended, which we know from the fact that Isolt Sayre named one of the Ilvermorny houses after them, and they definitely fit the Slytherin vibe, as they are a snake. They are mainly water-dwelling creatures, meaning that they would likely get on well with Annabelle, and be able to hang out with her in the lake, but they would also likely get on well with Basil the Basilisk, because they are both types of snakes. They are also very intelligent creatures, so I'm sure they'd fit well amongst the Slytherins!