Wow,
lots of possibilities, from ticks and mice
XMRV,
Theiler's murine encephalomyelitis virus (TMEV),
unknown:
De Freitas retrovirus, CAV
australian/danish retrovirus, whatsitsname,
from cows:
and Dogtorj posted on his facebook page about viruses not killed in heat-treated milk, about bovine leukemia virus like the mouse leukemia virus
cats:
feline leukemia virus