Monsters can only have up to three normal skills plus one additional secret one. They cannot learn new skills beyond the ones they already have.

Bear in mind that you can also poach excess Hydras for some nice items that will appear in Fur Shops.

Hyrudras:
Blood Sword (drain sword)
Scorpion Tail (a porwerful flail)

Hydras:
Setiemson (perfume that gives invisibilty and haste)
Rubber Suit (best normal clothing, gives immunity to lightning)

Tiamats:
Ryozan Silk (Dancer weapon)
Whale Whisker (a powerful stick weapon)