Charles Ellicott Commentary Leviticus 27:11

"And if it be any unclean beast, of which they do not offer an oblation unto Jehovah, then he shall set the beast before the priest;" — Leviticus 27:11 (ASV)

And if it be any unclean beast. —That is, if what he vows consists of an unclean beast, which does not belong to the three kinds of sacrificial quadrupeds, and which cannot therefore be sacrificed on the altar. According to the authorities during the second Temple, however, the expression “unclean beast” here denotes defective sacrificial animals, such as oxen, sheep, and goats with blemishes, which have become unlawful for the altar.