I don't know about the definition you're using, but taxonomically speaking there is at least one single-celled organism that's indisputably part of (as in desce

I don't know about the definition you're using, but taxonomically speaking there is at least one single-celled organism that's indisputably part of (as in descended from) Animalia: Canis lupus incertae sedis[1][2], a infectious disease that evolved from dogs[3]. (On further wiki-walking, there also appear to be similar diseases evolved from devils, hamsters, and clams[4].) There are also choanoflagellates[5], which might have reverted to single-celled life after a stint as multicellular, or might not, but are in any case more closely related than the opisthokont[6] common ancestor of animals and fungi. (These are rather more relevant to the context of "were there single-celled animal ancestors?". If opisthokonts already formed differentiated clonal colonies, then there was no point in the main [modern normal animals] lineage that was defensibly already animal but not yet multicellular. Apoikozoa[7] allegedly formed colonies, but it's not clear to what extent they were differentiated and/or obligate.) 1: https://en.wikipedia.org/wiki/Canine_transmissible_venereal_tumor https://en.wikipedia.org/wiki/Canine_transmissible_venereal_... 2: https://www.quora.com/Has-a-complex-organism-like-a-dog-ever-evolved-into-a-single-cell-organism/answer/Israel-Ramirez https://www.quora.com/Has-a-complex-organism-like-a-dog-ever... 3: https://sci-hub.se/https://doi.org/10.1016/j.gde.2015.03.005 https://sci-hub.se/https://doi.org/10.1016/j.gde.2015.03.005 4: https://en.wikipedia.org/wiki/Transmissible_cancer https://en.wikipedia.org/wiki/Transmissible_cancer 5: https://en.wikipedia.org/wiki/Choanoflagellata https://en.wikipedia.org/wiki/Choanoflagellata 6: https://en.wikipedia.org/wiki/Opisthokonta https://en.wikipedia.org/wiki/Opisthokonta 7: https://en.wikipedia.org/wiki/Apoikozoa https://en.wikipedia.org/wiki/Apoikozoa