Green’s Dictionary of Slang

1. a horse.

1913–14		Van Loan ‘For the Pictures’ in Taking the Count 336: Them dinky-legged beetles go good in the slop.
1917		Van Loan ‘A Morning Workout’ in Old Man Curry 213: ‘What horse is that?’ ‘This jus’ one ’em Curry beetles.’.
1938		D. Runyon ‘It Comes Up Mud’ in Runyon on Broadway (1954) 541: He is not a bad-looking beetle, though maybe a little [...] leggy.
1950		Goldin et al. DAUL 25/2: Beetle. [...] 2. A slow race horse. ‘Imagine a geezer on the make (a thief) throwing his iron (money) on the beetles!’.
1958		R. Prather Scrambled Yeggs 71: ‘Where from you know Cookie?’ ‘Back when I thought I could beat the beetles.’.

2. an unattractive (young) woman, esp. one who dresses in flashy clothes.

1933		J.T. Farrell ‘Wedding Bells Will Ring’ in Short Stories (1937) 213: If we went to a hop, we might not dance with Gloria Swanson, but we could find plenty of nice beetles to rub ourselves against.
1935		A.J. Pollock Und. Speaks n.p.: Beetle, an unappealing woman.
1944		P. Kendall Dict. Service Sl. n.p.: beetle . . . a girl to Alaska soldiers.
1949		Monteleone Criminal Sl. (rev. edn).
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 8: Beetle – an ugly woman.
2010		IOL News (Western Cape) 27 Sept. 🌐 As Paul McCartney later discovered, ‘Beetles’ is slang for motorcycle girls.

3. (UK juv.) a police officer.

1959

I. & P. Opie Lore and Lang. of Schoolchildren (1977) 395: Nicknames current among boys [...] Beetle, Beetle-crusher, Beat Basher.