Interesting those appear to be Fabuland heads on normal minifigure bodies, but the Fabuland minianimals were not modular like minifigs.

Fabuland was discontinued in 1989 and many elements have since reappeared in other product lines, such as Mickey Mouse, Harry Potter, Belville, Castle/Kingdoms, Friends, Lego Games and others. The Fabuland line was referenced in the 2014 film The Lego Movie.

https://en.m.wikipedia.org/wiki/Lego_Fabuland

It sounds like maybe the heads have been rereleased with other sets.