It is a matter of some dispute whether Felix was created by Sullivan or his top animator Otto Messmer.
Some animation historians accepted Messmer's claim, as he was the principal animator on the Felix series.

 
However, Sullivan was drawing cartoons for Paramount Magazine by 1919 and later when he signed a contract as an animator with Paramount Studios in March 1920, one of the subjects specified in his curriculum vitae was a black cat named Felix who had first appeared in Paramount Magazine as a character named "Master Tom" in a cartoon series named Feline Follies, tending to support Sullivan's claim definitively.

Firsthand accounts were recorded in print, notably a recollection from 1953 by Australian writer Hugh McCrae, who was sharing an apartment with Pat Sullivan just before Felix was created. '
It comes properly as a postscript that in New York McCrae shared a flat with Pat Sullivan, the famed creator of "Felix, the Cat."
When a film about Felix was being planned, Sullivan suggested that McCrae should do the drawings while he (Sullivan) supplied the ideas.
McCrae refused and has regretted it ever since.'

Australian cartoonists find the Messmer claim not credible.
Messmer came forward decades after Pat Sullivan's death, claimed Felix was his creation and placed the place of creation of the lucrative character in his own house, away from his boss's office.
He excluded Pat Sullivan completely, and yet the lettering throughout the creation matches Pat Sullivan's hand.
It is also telling that a cartoon kitten says "MUM" in Feline Follies, with the Australian/British spelling, rather than "MOM", the American spelling; Messmer is less likely to have written that.
It is a matter of some dispute whether Felix was created by Sullivan or his top animator Otto Messmer. Some animation historians accepted Messmer's claim, as he was the principal animator on the Felix series.
 
However, Sullivan was drawing cartoons for Paramount Magazine by 1919 and later when he signed a contract as an animator with Paramount Studios in March 1920, one of the subjects specified in his curriculum vitae was a black cat named Felix who had first appeared in Paramount Magazine as a character named "Master Tom" in a cartoon series named Feline Follies, tending to support Sullivan's claim definitively.
Firsthand accounts were recorded in print, notably a recollection from 1953 by Australian writer Hugh McCrae, who was sharing an apartment with Pat Sullivan just before Felix was created. 'It comes properly as a postscript that in New York McCrae shared a flat with Pat Sullivan, the famed creator of "Felix, the Cat." When a film about Felix was being planned, Sullivan suggested that McCrae should do the drawings while he (Sullivan) supplied the ideas. McCrae refused and has regretted it ever since.' 
Australian cartoonists find the Messmer claim not credible. Messmer came forward decades after Pat Sullivan's death, claimed Felix was his creation and placed the place of creation of the lucrative character in his own house, away from his boss's office. He excluded Pat Sullivan completely, and yet the lettering throughout the creation matches Pat Sullivan's hand. It is also telling that a cartoon kitten says "MUM" in Feline Follies, with the Australian/British spelling, rather than "MOM", the American spelling; Messmer is less likely to have written that.
Pat Sullivan (film producer)