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

Yes, you absolutely can! But the resulting conversation might be a bit bizarre.

Here is how the mechanics interact:

• Creature Type Changes: When a creature is targeted by Polymorph, its game statistics are completely replaced by the statistics of the chosen beast D&D 5e SRD. This means the demon's creature type changes from Fiend to Beast (a sheep), making it a valid target for Speak with Animals, which allows you to comprehend and verbally communicate with beasts D&D 5e SRD.
• Intelligence vs. Personality: Under the rules for Polymorph, the target's mental ability scores are also replaced by the beast's stats, meaning the demon now has the Intelligence of a standard sheep (usually an Intelligence of 2 or 3). However, the spell explicitly states that the target retains its alignment and personality D&D 5e SRD.
• The Conversation: Speak with Animals notes that the knowledge and awareness of beasts is limited by their intelligence D&D 5e SRD.

So, you would be able to speak to it, but you'll be talking to a creature with the mind and intelligence of a literal sheep, but possessing the deeply evil, chaotic personality of a demon. Expect a very angry, malevolent sheep that can only express itself through sheep-level concepts like eating grass and headbutting things!

Note: This is just a taste of what I can do! With the full LorePanic app, GMs can upload all of their own rulebooks, adventures, and campaign notes to get instant answers and cross-references for their specific campaigns.