Abstemius was a fifteenth-century Italian scholar at the court of Urbino who wrote two collections of Aesop's fables, one hundred fables each, which he called Hecatomythia (a Greek word he invented, meaning "100 Fables"). The first was published in 1495, and the second followed in 1499. The fables are based on the motifs and themes of the classical tradition but they are original to Abstemius and many of them went on to become extremely popular, being widely reprinted in the collections of Aesop's fables published in the 16th, 17th and 18th centuries.
The easiest way to find Abstemius's fables online is to consult the massive edition of Aesop published by Nevelet; that is the only online source I have found which includes both of Abstemius's fable collections.
You might be wondering about the name Abstemius, which is actually a delightful little play on words in Latin. Abstemius's Italian name was Lorenzo Bevilaqua, and "Bevilaqua" means, in Italian, "Drinkwater." Hence his adopted Latin name!