There is too much lore change from over 20 years ago + cinematics and what nots to give such things a concrete “would be ruleset” without turning it headcannon.
You get to see in wc3 Cairne get baited by a polymorphed squad of footmen into an ambush (so there they have self awareness)
Then in wow rpg lore this makes the “transfiguration” turn you into what ever mindhung creature it is. Somewhere along the lines between tbc and wotlk it was considered “illusionary magic” or even “Mind affecting” from time to time which meant a will save was prone to resist it and undead being less affected by it.
In one of the entry cinematics in wow directly you see a mage turn someone into a sheep and it plops down mid air flailing for air how ever self aware that its about to drop in a very pecculiarly “humane way” by flopping its arms around. If you’ve ever seen sheep jump of things, and believe me its rare, they aim to drop on their belly , and from lower altitude they just aim to land on their front hooves like the likes of goats.
Its one of those spells often only used on npc related encounters thats Dm to dm policed differently, with players often avoiding to use it on each other for the sole purpose of being unsure what to do afterwards.
In dalaran there is a book called “polymorphic rules and regulations” in wow, found in dalaran, and this is inside:
Rule# 1: Do not turn a creature of lesser intelligence into a creature of higher intelligence - Cerebral brain function rarely translates in a polymorph… But sometimes it does. Voracious creatures such as serpents and wolves need not have their predatory instincts honed with a human mind. We feel obligated to quote Small Claims Disaster Case 12651-B, Labeled, “Bartholomew: The Bear who Bears Arms.”
And the more amusing one rule 3 in that same book which mentions a “blue checkered box” which is the basic model loaded in game when you morph into a unknown model entry. (Do you want to canonize that?)
Alongside many other things mentioned in other books troought wow history. The book is the one that replaced the “schools of magic: transmutation” instead.
So take your pick, roll, or go with the flow.