No its is never possible in evolution, though toxins are available in some of the mammals, but they are quite different from insects as well as reptiles and fishes. Cross species transfers are impossible because in evolution reptiles were ad vented much earlier than mammals, but it is possible some modified precursor or any other molecular domain may be similar in both taxa. and both may have different functions. At the molecular level evolution is fine, but at the level of expression it is quite different and promoters are quite different. That is the main cellular difference among  similar species of the same group. That is to selection parameter.  Though there occurs overlapping in genes but no over lapping occurs in promoters as well as strong antigenic molecules. 

Even a venomous snakebite is not always fatal and might be the cause of a viral infection. A virus that is highly invasive and virulent might be a vector for the transfer of genetic material (such as a retroposon) from one species to another. I wonder which viruses are known to infect mammals as well as reptiles and what kind of genetic material could they possibly transfer.

Most likely there was some vector in this horizontal transfer - virus or bacteria are both possible.

A review by Schaack et al. (2010) summarises over 200 known cases of HTT (horizontal transposon transfer), twelve of which were between different phyla. In the TE-Thrust hypothesis (Oliver & Greene 2011; 2012) HTT is regarded as an important part of the life cycle of TEs  as they generally accumulate mutations and eventually become non-viable in the genomes they occupy, and this can down grade the efficacy of TE-Thrust. However, they are sometimes enabled, via chance events, to periodically make fresh starts with fully functional elements in the genomes of other lineages, and TE amplification generally occurs immediately after HTT.

Viruses and bacteria appear to be likely vectors of HTT, but other possible vectors have been proposed, such as endoparasites and intracellular parasites (Schaack et al. 2010) and HTT has been proposed as a major force driving genomic variation and biological innovation by Schaack et al. (2010) and Oliver & Greene (2011, 2012) and PhD Thesis KR Oliver (2012).

References

Schaack S Gilbert C & Feschotte C 2010 Promiscuous DNA: horizontal transfer of transposable elements and why it matters for eukaryote evolution. Trends in Ecology and Evolution 25: 537-546.

Oliver KR & Greene WK 2011 Mobile DNA and the TE-Thrust hypothesis: supporting evidence from the Primates. Mobile DNA 2:8. 

Oliver KR & Greene WK 2012 Transposable elements and viruses as factors in adaptation and evolution: an expansion and strengthening of the TE-Thrust hypothesis. Ecology and Evolution doi: 1002/ece3.400 

PhD Thesis, Oliver KR 2012 (Murdoch University) The Dynamic Eukaryote Genome: Evolution, Mobile DNA, and the TE-Thrust Hypothesis

Piskurek O & Okada N 2007 Poxviruses as possible vectors for horizontal transfer of retroposons from reptiles to mammals. PNAS 104:12046-51.

http://www.ncbi.nlm.nih.gov/pubmed/17623783