BUG: Pets can't learn certain skills

Problem

• Cats & dogs can't learn hunting
• Horses can't learn racing

Cause
This might be a conflict with the spell book but that's not 100% certain. It could also be the same skill modifier bug that made Sims not learn the athletic skill anymore and Vampires learn at insane rates.

Solution

• Uninstall the spell book and delete your Cache files. If that does not work, start a new game and check if it's working there. It might be that if you already have the bug in a game, you can't get rid of it.
• Use the cheat testingcheatsenabled true and shift-click on the mailbox. Then choose make needs static (Credit goes to SiriusNSassy1176)

Mods that solve the problem

• Overwatch, V41 or newer fixes this issue. 

Further Reading

• BUG: Sim can't improve theirs skills / Vampires learn insanely fast