Musar su Genesi 26:13

I shall now explain the meaning of the words: עד כי גדול מאד "until he was very great," in 26,13. Bereshit Rabbah 64,7 on that verse states that when people wanted to describe excessive wealth they used to describe it in terms of the silver and gold possessed by Avimelech. After Isaac had become rich, they described excessive wealth in terms of the dung of Isaac's mules. This seems a curious comparison. How can we assume that Avimelech's wealth did not even amount to the value of the dung of Isaac's mules? Another difficulty is the Midrash's comment on the words מצא את הימים, in Genesis 36,14. These yeymim are described by Bereshit Rabbah 82,15 as half-donkey and half-horse. Anah who crossbred horse and donkey to produce mules is severely criticized for interfering with G–d's plan of maintaining the purity of the species. How can we assume that Isaac a) kept or raised such animals, b) was the first one to crossbreed, seeing that the Torah reports this as something new much later and in Jacob's lifetime?