Additional to the ones mentioned here and throughout the posts, the followings links and literature may be mentioned as an update and further source of material.
Medieval Animal Data Network
The international Medieval Animal Data-Network (MAD) was founded in 2005 at the Department of Medieval Studies at Central European University (Budapest) as an attempt to bring together multi-stranded intellectual thought concerning human-animal relationships in the European Middle Ages. The aim is to explore the exploitation of animals by documenting and analyzing archaeozoological, textual and visual data.
This is a meta-site containing several interesting links mentioned below.
This site contains bibliographical references about snails in medieval texts. Last modified in 2007.
The link brings you to the online archive of the Institut für Realienkunde, Universität Salzburg. When search for “Tiere” > “Schnecke”, 10 results are presented with very detailed data and annotations, but very low resolution pictures.
Evans (1896) described in detail the known animal symbolisms in ecclesiastical architecture, mentioning several occurrences of snails . One of these is this picture of a stained window (16th century) in the Saint-Nizier church, Troyes, France, showing a horned snail with also two tentacles (Evans, loc. cit.: 81). The shell is not visible and, as far as seen till now, it is the only reference to a horned snail; a true Apocalyptic monster!
 Evans, E.P. (1896) Animal symbolism in ecclesiastical architecture. Heinemann, London, 173 pp. PDF