And the PH says that if you come to your next action and have not taken your readied action, you lose it.
OK, I saw that part in the PHB and understand it (I think!), but to me that's different than being able to cancel an action. To me, the above says, "If the pet doesnt charge you before your next initiative, your readied action didnt get used and thus was wasted." But, in the example I gave, I never came to my "next action", because the pets charge took place while my "readied action" was still valid.
You either take your still readied partial action, or wait till your normal initiative and give him a flurry.
I dont think that I have the option of flurrying the pet on my normal initiative. Dont get me wrong; I'd like to find support for the option of cancelling a readied action that turns out to be suboptimal/suicidal.
If its true that I can cancel, then I dont feel like I've totally lost out on Round 2 - I can make a decision during Round 2 and try and adjust to the changing situation. If I cannot cancel, I dont get to make a decision and events overtake my poor vulnerable Monk. (It wouldnt be so bad if it was just me vs. the pet, its the idea that a combat with multiple friends and foes swirls around me and I am kinda frozen in time by my readied action.)