"This is not an ordinary cat."
Translation:Ini bukan kucing biasa.
In general, 'bukan' is used to negate a noun (or wordgroups).
'tidak' is used to negate an entire sentence or to negate a predicate when the predicate is a verb/adjective/numeral phrase.
Here is a Tinycards deck with examples :
It should be tidak in this sentence. It is "ordinary" that is being qualified not "cat". Ordinarily tidak qualifies verbs so at the very least tidak should be allowed as well as bukan. Its a 'grey' area where adjectives are concerned as it is here.