Let's switch to encounter time scale and call this Turn Zero.
Willie is on top of the wagon with his sling and has the best view.
Chegs is on the ground with his warhammer and shield.
Tet picks himself up and is ready for battle. (His turnip-related injury is annoying, but not crippling.)
Dougal rolls 1d6=5 (he needed a 4) and successfully uses his thief class ability for additional stealth above and beyond his racial ability. He's well-hidden in the undergrowth northwest of the cart.
Potty and the Farmer are with the oxen. Potty casts Speak with Animals. It takes him the full turn to cast the spell. According to the Player's Handbook, the spell affects "one animal type" with a duration of 2 rounds/level. I rule that Potty will be able to Speak with Ungulates starting at the very end of Turn Zero and continuing throughout Turns 1-2.
As the spell takes effect, Potty hears the oxen Burr and Dock repeating over and over: PALE FOREST-OXEN! PALE FOREST-OXEN!
Willie catches a glimpse of pure white, the color of fresh snow or bleached bones. It appears to be a large solitary beast, but it's difficult to make out its exact form through the thick forest. Willie sees that it is a 4-legged creature, standing 8 to 10 feet tall. Despite its immense size, the animal does not lumber noisily through the forest, but rather picks its way through the trees with delicate grace. Lucky thing it didn't sneak up on our Heroes completely unawares! At its current speed and trajectory, the Pale Forest-Oxen will approach within sling range, revealing its true form, on Turn 1.
What now, Hobbetses?