I think the bonus roll for the Viscous Horror applies only to normal and heroic mode since that's where it normally drops from.
Unless they made all ToT pets available in every version of the raid, the Viscous Horror will be limited to its designated loot table (normal and heroic), and I haven't heard of any information that they've made any changes to the loot tables. So it's very likely that LFR will still only yield the Living Fluid (drop or bonus roll).
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, my vanity pet collecting blog!