There are two subtly different interpretations as to defaults:
- You can use one skill for actions normally done with another skill. (I'm not a skilled rider, but my general training with animals allows me to get the animal to cooperate with me enough to ride it)
- Training in one skill provides a basis to practice another skill. (My general training with animals is sufficient that I effectively am a somewhat skilled rider)
The first probably would not justify adjusting for attribute differences. The second probably would. You might even switch between them on a case by case basis. Maybe use an IQ-based Animal Handling default for avoiding problems on a long ride, and a DX-based default to handle a mount in a whirling melee.