Well, we shall start with a list of ingame icons for now, if I find something that really really
really suits a certain attack and is too awesome to not use, I'll let you know. Until then...:
Tackle: Big Bad Voodo under abilities? (Seems awkward, but actually could go well...)
Scratch: Claws of Attack under items
Water Gun: Orb of Frost under items (might need to find a more suitable import

)
Vine Whip: Poison Sting icon under abilities (unfortunately a passive icon, so an alternative might be preferable here)
Poison Powder: ReplaceableTextures\CommandButtons\BTNDryadDispelMagic.blp (this is the path for the hidden icon)
Sleep Powder: Sleep under abilities (will do for now, anyway)
Stun Spore: Doom under abilities (will do for now, anyway)
Ember: Orb of Fire under items
Growl: Battle Roar under abilities (might not be that suitable, but will give the right idea)
Bite: Cannabilize under abilities
Withdraw: Spiked Carapace under abilities (use the research one)
Rage: Stampede under abilities
Harden: ReplaceableTextures\CommandButtons\BTNEntanglemine.blp (this is the path for the hidden icon)
Tail Whip: Ray of Disruption? (Could work if there are no better alternatives)
String Shot: ReplaceableTextures\CommandButtons\BTNPoisonArrow.blp (this is the path for the hidden icon)
Confusion: Detonate under abilities (looks pretty good to me ;D)
Fury Attack: Quill Spray under abilities (I think the Turn Off icon for this ability would be the better of the 2 options you will find)
Poison Sting: Envenomed Spears under upgrades (the research version would look better as it is not a passive icon)
Blizzard: Freezing Breath under upgrades
Over Heat: Flame Strike under abilities
Well, I think that covers all the attacks you'll need for now + a couple extra

. Hope you like my suggestions!