Homosexual behavior has been observed in a significant number of animal species. A 2019 paper states that same-sex sexual behavior has been observed in over 1,500 species[1]. However, it's important to note that the term "homosexuality" as applied to animals can encompass a range of behaviors, including courtship, sexual activity, pair-bonding, and parental activities[1].

In terms of specific percentages, it varies widely among species. For instance, about 10% of rams (male sheep) refuse to mate with ewes (female sheep) but do readily mate with other rams[1]. Another source suggests that as many as 8% of rams demonstrate sexual attraction to other rams[2]. Among bonobos, roughly 60% of all sexual activity occurs between two or more females[1].

It's also worth noting that in many instances, the homosexual behavior is part of the animal's overall sexual behavioral repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans[1].

In conclusion, while it's difficult to provide a single percentage that applies to all animals, it's clear that a significant number of species exhibit some form of same-sex sexual behavior. The percentage of individuals within a species that exhibit such behavior can vary widely.

Sources
[1] Homosexual behavior in animals - Wikipedia https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals
[2] How Common Is Homosexuality in Animals? - ThoughtCo https://www.thoughtco.com/homosexuality-in-animals-4164365
[3] Why Is Same-Sex Sexual Behavior So Common in Animals? - Scientific American Blog https://blogs.scientificamerican.com/observations/why-is-same-sex-sexual-behavior-so-common-in-animals/
[4] 1,500 animal species practice homosexuality - News-Medical.net https://www.news-medical.net/news/2006/10/23/1500-animal-species-practice-homosexuality.aspx
[5] Scientists explore the evolution of animal homosexuality - Imperial College London https://www.imperial.ac.uk/news/190987/scientists-explore-evolution-animal-homosexuality/
[6] 10 animal species that show how being gay is natural – DW – 08/02/2017 https://www.dw.com/en/10-animal-species-that-show-how-being-gay-is-natural/g-39934832

By Perplexity at https://www.perplexity.ai/search/0d6061d7-b401-4da7-bb90-c953503c2d95