...Aboleth and 16 others.
Here's a list of all the A entries in my B/X Monster Reference Index. I plan to add a list for each letter of the alphabet between now and the end of the Kickstarter campaign.
Aboleth, Adherer, Aerial Servant, Afanc, Algoid, Ankheg, Ant (Giant), Ant Lion, Ape (Albino), Ape (Chimpanzee), Ape (Gorilla), Ape (Man-Eating), Apparition, Archer Bush, Ascomoid, Assassin Bug (Giant), and Axe Beak.
The index has only had minimal proofing so far, but if you spot something that is mispelled or out of alphabetical order, feel free to let me know.