A fairly large citrus fruit resulting from the hybridization of a frua mandarin and a sweet pomelo (hence 'mandelo'). Despite the common name 'cocktail grapefruit,' it is not actually a true grapefruit.

https://www.diversivore.com/ingredient-pages/mandelo-cocktail-grapefruit/

The mandelo was developed in the 1950s at the UC Citrus Experiment Station, but then escaped into public orchards. It is a yellow-orange fleshed tri-specific citrus hybrid between a 'Frua' hybrid mandarin ('Dancy' mandarin × 'King tangor')and a Siamese Sweet pomelo. This makes it 1/4 King tangor, 1/4 Dancy tangerine, and 1/2 Siamese Sweet Pomelo. It is named for its ancestors, mandarin and pomelo.

https://en.m.wikipedia.org/wiki/Mandelo