。
The snail smoothly extended its horns.
Literal
Snail [subj-が] smoothly horns [obj-を] put-out.
すっと is a mimetic adverb for a smooth, swift, effortless motion — the snail's tentacles gliding out. 角を出す 'to put out horns/tentacles' is the literal and standard description. カタツムリ 'snail' is in katakana here (biological convention for species names). A clean, visual observation sentence.