While not really an endogenous retrovirus capable of causing disease by itself, this occurs to an extent in cats. In cats, Feline Leukemia Virus (FeLV) is present as both an exogenous retrovirus, and integrated into the genome several times as an endogenous retroviruses. Several strains of FeLV Subtype B occurs when the "env" region of endogenous FeLV recombines with exogenous FeLV. This "new" virus is not generally considered transmissible, but causes cancers to develop in the cat.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8387034/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7152252/