Which species has the highest proportion of homosexual individuals?
Homosexual behavior have been observed more than 1500 species (according to Museum of Natural History of the University of Oslo). A full comparison between all known species is far beyond the scope of this site, but below I discuss three species of interest. Also, the details of what counts as an homosexual behavior, and also whether you compute incidence or prevalence makes the whole discussion semantically complicated.


According to Simon Levay and Aldo Polani, in domesticated sheep, 10% of the rams (males) prefer mating with other rams than with ewes (females).
About 25% of black swans are homosexuals (Braithwaite, 1981).
In lions, 8% of the mounting is in between two males (Bagemihl 2000)


In humans, around 3%-5% of the surveyed people claim being homosexual or bi-sexual (see wiki > demographics of sexual orientation#Modern_survey_results).