"It is the hundredth dog."
Translation:E al o sutălea câine.
You have to use the article "al" because the related noun ("câine") is masculine (you use the same article for neuter nouns), as opposed to the form "a" which is used for counting feminine nouns.
The word "o" comes from the numeral "o sută" (one hundred). Unlike English, Romanian does not drop the "o" (= one) part in the ordinal form of the number: al o sutălea (m, n), a o suta (f) = the hundredth.