Green’s Dictionary of Slang

softhorn n.

[a donkey has soft ears rather than horns]

1. a donkey, an ass.

[UK]Farmer & Henley Sl. and Its Analogues.

2. a simpleton, a fool.

[US]T. Haliburton Clockmaker I 269: I allot [...] that the blue-noses are the most gullible folks on the face of the airth — rigular soft horns, that’s a fact.
[US]T. Haliburton Clockmaker III 58: He thinks he has a soft horn to deal with.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]Hotten Sl. Dict.
[UK]Sl. Dict.

In phrases