As in 'Here is my new kindergarten class, all bright eyed and bushy tailed.' The allusion here is to the appearance of a squirrel, which with its beady eyes and bushy tail looks ready for anything. 1930s
A unicorn-tailed soft bottom is a person who is a designated bottom in the furry community. This individual will insert a soft-tailed butt plug into their anus and engage in sexual activities, mainly double anal penetration.
p1 - "Hey Gerard! Did you hear about Dillon? He's a UTSB!"
p2 - "That furry faggot? What's a UTSB??"
p1- "He's a unicorn-tailed soft bottom now, he likes to wear butt plugs with fancy tails on them while getting ass fucked by his friends."