John Gill Commentary Leviticus 27:26

"Only the firstling among beasts, which is made a firstling to Jehovah, no man shall sanctify it; whether it be ox or sheep, it is Jehovah`s." — Leviticus 27:26 (ASV)

Only the firstlings of the beasts
These are excepted from being sanctified, or set apart for sacred uses, for a very good reason, suggested in the next clause:

which should be the Lord's firstling, no man shall sanctify it ;
it being what he has a claim upon, and ordered to be sanctified to him by a law previous to this, (Exodus 13:2) ; wherefore to sanctify such a creature, would be to sanctify what was his before; not merely in a general sense, in which all creatures are his, but in a special sense, having in a peculiar manner required it as his; and therefore to sanctify, or vow to him, what was his before, must be trifling with him, and mocking of him:

whether [it be] ox, or sheep ;
the firstlings of either of them:

it [is] the Lord's ;
which he has claimed as his own special and peculiar property, antecedent to any vow of its owner.