Is it true that oral sex exists in non-human animals?
Yes, it is true. Have a look at this post:9 Animals Who Are Into Oral Sex:


  A recent study of fruit bats found cunnilingus to be a major part of their repertoire. While following a colony of 420 bats roosting in a fig tree over the course of 13 months, researchers witnessed 57 incidences of sex — both oral and intercourse. The female bats who received oral sex before intercourse were found to last longer during copulation. 


The paper they cite is this one.