Can you use Speak with Animals to talk to a demon that has been polymorphed into a sheep?

Actually, according to the strict reading of the rules, no, you cannot!

Here is how the mechanics interact:

• Polymorph: When a creature is targeted by Polymorph, its game statistics are replaced by the stat block of the chosen Beast form. However, the exact wording specifies that "the target retains its alignment, personality, creature type, Hit Points, and Hit Point Dice" D&D 5e SRD 5.2 — Spe.... This means a demon polymorphed into a sheep is technically still a Fiend, not a Beast.
• Speak with Animals: This spell specifically states that you "can comprehend and verbally communicate with Beasts" D&D 5e SRD 5.2 — Spe....

Because the sheep is still technically a Fiend instead of a Beast, Speak with Animals would have no effect on it! The spell's magic simply wouldn't recognize the demon as an animal.

(Fun twist: If the demon was transformed using a 9th-level True Polymorph spell, it only retains its Hit Points, Hit Point Dice, alignment, and personality—meaning its creature type actually would change to Beast, and Speak with Animals would work!)

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