Short answer: The sentence is grammatically correct.
Original: You guessed it! It is the shadow of a jerboa.
Notes and why it works
- "You guessed it!" is an interjection that shows surprise or confirmation. The exclamation mark is fine here.
- "It is the shadow of a jerboa." is a complete sentence: 'it' is the subject, 'is' is the verb, and 'the shadow of a jerboa' is the predicate. The definite article 'the' is correct because you mean a specific shadow (for example, the one in a picture).
- "jerboa" is a common noun (a small jumping rodent), so it stays lowercase.
More natural, conversational option
"You guessed it! It's the shadow of a jerboa."
Using the contraction "it's" (short for "it is") sounds more natural in speech and in informal writing.
Other alternatives depending on tone
- Casual: "You guessed it — it's the shadow of a jerboa." (uses an em dash to connect the two ideas)
- Neutral/formal: "You guessed correctly. It is the shadow of a jerboa."
Quick tips
- Use "it's" for informal speech or chat; use "it is" for more formal writing.
- Keep the exclamation if you want excitement; use a period for a calmer tone.
Any of these small changes are a matter of style; grammatically, your original sentence is fine.