What was the abomination of the Egyptians?
NWT Exodus 10:25, 26 "But Moses said: “You yourself will also provide us with sacrifices and burnt offerings, and we will offer them to Jehovah our God. 26 Our livestock will also go with us. Not an animal will be allowed to remain, because we will be using some of them to worship Jehovah our God, and we do not know what we will offer in worship to Jehovah until we arrive there.”
The Jews would offer cattle as sacrifices to Jehovah which would be a very offensive in the eyes of the Egyptians as they held cattle as sacred to the Gods, for example, Rams sacred to Amon; Cows sacred to Hathor, thus an "abomination" to them. More below:-
"Ancient Egyptians did not adore animals as simply pets. Certain animals were considered gods. The Egyptian was taught to pay a religious regard to animals. In one place goats, in another sheep, in a third hippopotami, in a fourth crocodiles, in a fifth vultures, in a sixth frogs, in a seventh shrew-mice. They were sacred creatures, to be treated with respect and honour, and under no circumstances to be slain, under the penalty of death to the slayer.
Besides this local animal-cult, there was a cult which was general. Cows, cats, dogs, ibises, hawks, and cynocephalous apes, were sacred throughout the whole of Egypt, and woe to the man who injured them! A Roman who accidentally caused the death of a cat was immediately "lynched" by the populace.
Inhabitants of neighbouring villages would attack each other with the extreme anger if the native of one had killed or eaten an animal held sacred in the other.
In any house where a cat or a dog died, the inmates were expected to mourn for them as for a relative. Both these and the other sacred animals were carefully embalmed after death, and their bodies were interred in sacred repositories.
Now consider some lucky bulls, known as Hapi of Apis, who lived in Memphis around 1650 B.C. This type of bull was believed to be the actual incarnation of the god Phthah.
The Apis bull dwelt in a temple of his own, near the city. He had his train of attendant priests, a group of cow-wives, his meals of the most exquisite food, his grooms who kept his coat clean and beautiful, his chamberlains who made his bed, his cup-bearers who brought him water. And on scheduled days, he was led in a festive procession through the main streets of the town, so that the inhabitants might see him, and come out of their dwellings to revere him. When he died he was carefully embalmed, and deposited, together with magnificent jewels and statuettes and vases. He was placed in a polished granite sarcophagus, cut out of a single block, and weighing between sixty and seventy tons. ..."-http://history.intellichristian.com/ancient-egyptians-animal-worship-to-the-extreme