Rainbow Dash's wing injury a while back, forcing her to live like an Earth pony for several days, served as a huge epiphany. This episode brings her interest in reading full circle.
An ahuizotl, according to the legends, is a water-dwelling ambush predator. 800 years of unseasonable heat is almost certainly going to cause a drought. During a drought, there is less water to go around. When there's less water to go around, more animals will come from much further away to find any water, and will likely ignore any dangers there. Ahuizotl was essentially guaranteeing himself 800 years of free food.
