Nouns ending in -ail, -eil, -euil, or -ueil are masculine.
The whole family: -ail, -eil, -euil, -ueil. All masculine.
Is bétail masculine or feminine?
lebétail
The rule behind it
100% of -ail / -eil / -euil / -ueil nouns are masculine
In context
Un cow-boy conduit le bétail vers le pâturage.
A cowboy is driving cattle to the pasture.
Les cow-boys travaillaient ensemble pour rassembler le bétail.
The cowboys worked together to corral the cattle.
La sécheresse affame le bétail dans la région.
The drought is starving the region's livestock.