"He saw an animal."
Translation:Mi wnaeth e weld anifail.
Is it actually incorrect to drop the "mi"? I learned past tense without, just "wnes i", "wnest ti" and so on.
Mi and Fe used as pre-verbal particles in positive statements are common only in some dialects nowadays. The mutation of gwnes i, etc to wnes i, etc even without a preceding mi/fe is quite common in speech, though.
You can use it, and it is one of the accepted answers in the database - Gwelodd e anifail.